REESE  LIBRARY 


UNIVERSITY  OF  CALIFORNIA. 


ELECTRICITY  flND  MflGNETISM 


LESSONS 


NATIONAL  SCHOOL  OF  ELECTRICITY 


PREPARED  UNDER  THE  IMMEDIATE  DIRECTION 
OF  THE  FACULTY 


QENERAL  COURSE 


SECOND  EDITION,  REUISED 
FIFTH  THOUSAND 

PUBLISHED  BY 

CHICAGO  SCHOOL  OF  ELECTRICITY, 

335  Dearborn  Street, 

CHICAGO. 

1896. 


Copyright,  1895 


DONOHUE  &  HENNEBEKRY, 

PRINTERS,  BINDERS  AND  ENGRAVERS. 

CHICAGO. 


INTRODUCTION. 


YOUNG  men  and  women,  and  older  ones  too,  who  have  time 
and  money  at  their  disposal,  who  have  been  blessed  with  a 
sufficient  preparatory  education,  and  who  desire  to  learn,  some- 
thing about  electricity  either  for  pleasure  or  profit,  may  do  so  by 
entering  and  taking  the  very  comprehensive  courses  at  the  colleges 
and  technical  schools.  But  there  are  many  of  our  brightest  mechan- 
ics, artisans  and  business  men  who  have  not  had  the  benefits  of  a 
sufficient  primary  education,  especially  in  mathematics,  to  enable 
them  to  pass  the  more  or  less  exacting  entrance  examinations  required 
by  the  technical  schools  and  colleges,  and  these  and  many  others  also 
may  not  be  able  to  part  with  the  time  or  money  necessary  to  secure 
these  benefits.  It  is  to  these  classes  of  people  that  the  National 
School  of  Electricity  especially  appeals,  and  to  meet  their  require- 
ments, several  conditions  must  be  met. 

ist.  The  course  of  instruction  must  be  of  an  eminently  practical 
character  so  that  the  knowledge  acquired  may  be  immediately  utilized. 

2d.  The  course  must  be  furnished  at  times  and  places  con- 
venient to  participants  during  their  leisure  hours  ;  as  they  cannot  go 
to  school  it  is  necessary  that  the  school  be  brought  to  them. 

3d.  As  it  is  not  always  expedient  to  have  graded  courses,  it  is 
lecessary  to  bring  the  course  of  study  within  reach  of  those  of  even 
>ry  limited  education,  yet  have  it  include  all  that  may  be  essential 
lor  the  interest  of  advanced  students. 

4th.  In  order  that  the  theories  and  practice  of  electricity  may 
be  taught  together,  it  is  necessary  to  employ  actual  working  apparatus 
for  the  purposes  of  demonstration  at  all  stages  of  the  work. 


The  lessons  of  the  school  were  prepared  to  meet  these  conditions, 
by  Prof.  Dugald  C.  Jackson,  with  the  assistance  and  suggestions  of 
other  workers  of  the  faculty  of  the  school.  The  course  of  lessons  has 
now  been  thoroughly  revised  and  many  additions  have  been  made 
where  the  advances  of  the  science  of  electricity  and  its  application 
have  made  it  desirable,  and  some  alterations  suggested  by  the  experi- 
ence of  the  instructors  of  the  school,  have  been  incorporated.  As 
a  large  new  edition  of  the  lessons  is  now  being  issued,  and  a  consider- 
able demand  for  complete  sets  of  the  lessons  bound  in  permanent 
form  has  arisen  among  the  members  of  the  classes  of  the  school,  this 
volume  is  issued  in  response  to  this  demand.  The  volume  contains 
the  lessons  of  the  general  course  of  the  school  as  they  are  used  in 
the  instruction  of  the  classes. 

NOVEMBER,  1895. 


CONTENTS. 


LESSON  I.                                                         PAGE. 
The  Nature  and  Properties  of  Electricity 1 

LESSON  II. 
Machines  for  Generating  Electricity  by  Friction  and  by  Electric  Induction 9 

LESSON'  III. 
Electric  Batteries,  or  Appliances  for  Generating  Electricity  by  Chemical  Action.     15 

LESSON  IV. 
Electric  Batteries,  or  Appliances  for  Generating  Electricity  by  Chemical  Action 

(Concluded} 21 

\ 

LESSON  V. 
The  Nature  and  Properties  of  Magnetism.     Magnetic  Fields 27 

LESSON  VI. 
The  Magnetic  Effects  of  Electric  Currents,  and  Magnetic  Circuits 35 

LESSON  VII. 
Ohm's  Law  of  the  Flow  of  Electricity 42 

LESSON  VIII. 
Heating  Effects  of  Electric  Current.     Miscellaneous  Effects  of  Electric  Currents.     51 

LESSON  IX. 
Galvanometers  and  Voltameters 58 

LESSON  X. 
Measurement  of  Electrical  Resistance 66 

LESSON  XI. 
Every-day  Measurements  of  Electric  Currents  and  Pressures 74 

LESSON  XII. 
Every-day  Measurements  of  Electric  Power.     Condensers  and  the  Measurement 

of  their  Capacity 84 

LESSON  XIII. 
Electrolytic  Deposition  of  Metals 91 

LESSON  XIV. 
The  Electric  Telegraph.... 102 

LESSON  XV. 
Multiple  Telegraphy 110 


LESSON  XVI. 
The  Telephone 118 

LESSON  XVII. 
The  Construction  of  Telegraph  and  Telephone  Lines  and  Instruments 125 

LESSON  XVIII. 
Testing  Lines  for  Insulation  and  Conductivity  and  the  Location  of  Leaks  and 

Breaks 138 

LESSON  XIX. 
Principles  of  Continuous  Current  Dynamos  and  Motors 140 

LESSON  XX. 
Principles  of  Continuous  Current  Dynamos  and  Motors:     Their  Construction, 

Care  and  Attendance 155 

LESSON  XXI. 
Arc  Lighting  and  Arc  Light  Machinery 167 

LESSON  XXII. 
Incandescent  Lighting  and  Power  Transmission:     Two,  Three  and  Five  Wire 

Systems  of  Distribution  for  Electric  Lights  and  Motors 177 

LESSON  XXIII. 
Construction  of  Electric  Light  and  Power  Circuits  and  Their  Testing 191 

LESSON  XXIV. 
Testing  Electric  Light  Circuits,  and  the  Distribution  and  Measurement  of  Light.  205 

LESSON  XXV. 
Electromagnetic  Induction 213 

LESSON  XXVI. 
Alternating  Currents ., 220 

LESSON   XXVII. 
Alternating  Currents  and  Alternating  Current  Machinery.     (Concluded} 228 

LESSON  XXVIII. 
Miscellaneous  Applications  of  Electric  Motors 241 

LESSON  XXIX. 
Electric  Railways 256 

LESSON  XXX. 
Methods  of  Handling  and  Controlling  Railway  Motors  and  Generators , 266 

LESSON  XXXI. 
Model  Electric  Plants 279 

LESSON  XXXII. 
Underwriters' Rules,  Etc 297 

LESSON  XXXIII. 
Electric  Welding,  Forging,  Etc.     Electricity  Applied  to  the  Kitchen 304 

LESSON  XXXIV. 
Electro  Therapeutics 316 


The  National  School  of  Electricity, 


LESSON  I. 


THE  NATURE  AND  PROPERTIES  OF  ELECTRICITY. 

The  exact  nature  of  the  electricity  which  makes  itself  evident 
in  so  many  ways  has  never  been  determined.  Many  surmises  or 
theories  have  been  advanced,  but  none  have  yet  been  able  to  fully 
stand  the  test  of  close  examination.  By  experimental  evidence, 
which  has  been  gathered  for  decades,  we  have  been  able  to  deter- 
mine the  laws  which  govern  the  action  of  electricity,  though  we  do 
not  know  its  constitution,  very  much  as  we  know  the  results  of  the 
laws  of  gravitation,  though  we  do  not  know  what  "gravity"  really  is. 

The  derivation  and  use  of  the  word  "electricity"  has  itself  had 
a  development  parallel  with  that  of  the  experimental  development  of 
the  science  which  bears  its  name.  Springing  from  the  Latin  name 
for  amber,  electricus  or  electrum,  the  adjective  "ELECTRICAL" 
was  first  used  by  Dr.  Gilbert  in  a  book  published  in  1600  to  desig- 
nate the  attraction  for  light  bodies  like  chaff  and  bits  of  .paper 
which  amber  and  similar  substances  exhibited  when  briskly  rubbed. 
The  original  discovery  of  this  electrical  property  is  often  attributed 
to  a  Greek  philosopher  named  Thales,  who  lived  about  600  years 
before  the  Christian  era,  and  whose  writings  contain  the  earliest 
records  of  its  observation  which  have  come  down  to  us.  It  is  prob- 
able, however,  that  a  knowledge  of  this  peculiar  property  of  amber, 
and  possibly  of  other  bodies,  was  one  of  the  well-guarded  secrets  of 
the  priesthood  of  that  day. 

From  the  word  electrical  came  the  word  "ELECTRICITY."  Since 
the  day  Dr.  Gilbert  first  applied  the  word  electrical  to  a  particular 
phenomenon,  our  knowledge  of  all  the  sciences  has  widened,  and 
with  the  widening  has  come  an  equal  advance  in  the  knowledge 
which  was  represented  to  the  ancients  by  that  one  peculiar  property 
of  amber  and  similar  bodies.  The  term  electricity  is,  therefore, 
applied  not  only  to  one  little  branch  of  a  great  science,  but  covers  a 
vast  field  of  facts  which  are  supposed  to  be  based  on  the  same  under- 
lying cause. 

The  action  of  electricity  led  many  experimenters  after  Gilbert 
to  the  belief  that  it  was  a  fluid  not  perceptible  to  the  senses.  Our 


own  great  philosopher  and  statesman,  Benjamin  Franklin,  assumed 
it  to  be  a  fluid,  and  bodies  which  exhibited  electrical  manifestations 
were  thought  by  him  to  contain  either  more  or  less  than  a  natural 
amount  of  the  fluid.  A  Frenchman  named  DuFay  and  an  English- 
man named  Symmer  considered  electricity  to  be  composed  of  two 
fluids,  which  were  contained  in  neutral  bodies  in  equal  amounts. 
When  by  any  means  this  equality  was  disturbed  in  a  body  electrical 
manifestations  occurred. 

We  will  not  at  this  time  further  discuss  the  nature  of  electricity, 
but  will  pass  on  to  a  consideration  of  its  properties.  The  study  of 
these  properties  will  be  divided  into  two  classes — the  first,  in  which 
STATICAL  ELECTRICITY,  or  electricity  at  rest,  is  considered,  and 
the  second,  in  which  CURRENT  ELECTRICITY,  or  electricity  in 
motion,  is  considered.  There  is  no  well-defined  division  between 
these,  and  the  laws  governing  the  two  classes  are  practically  the 
same.  In  general,  however,  the  first  class,  or  static  electricity, 
includes  the  phenomena  known  by  the  ancients  where  electricity  is 
produced  by  rubbing  or  by  the  influence  of  one  ELECTRIFIED 
body  on  another.  The  second  class  includes  electricity  produced  by 
the  electric  batteries  and  dynamos  which  are  so  well  known  today. 
The  first  class  is  of  comparatively  small  importance  and  will  receive 
only  such  attention  in  the  earlier  lessons  as  is  necessary  on  account 
of  its  bearing  on  the  second  class. 

If  a  rod  of  sealing  wax,  amber,  or  other  resinous  substance  be 
rubbed  with  dry  wool  or  fur,  it  immediately  gains  the  property  of 
attracting  to  itself  light  bodies,  such  as  pith.  After  these  bits  of  pith 
have  been  in  contact  with  the  rubbed  body  for  a  short  time,  they 
usually  fly  off  as  though  repelled,  and  they  also  seem  to  repel  each 
other.  The  rubbed  body  when  in  this  condition  may  be  found  by 
proper  examination  to  be  covered  with  an  apparent  layer  of  electricity, 
which  is  called  a  CHARGE,  and  the  pith  balls  which  have  touched  it 
are  also  said  to  be  charged.  If  now  a  glass  rod  be  rubbed  with  silk 
it  will  show  the  same  properties  as  the  resinous  substance.  If  the 
glass  rod  be  brought  close  to  the  pith  balls  which  have  been  in  con- 
tact with,  and  repelled  by,  the  resin  rod,  it  will  strongly  attract 
them,  and  in  the  same  way  the  resinous  rod  attracts  the  pith  balls 
which  have  been  charged  by  contact  with  the  glass.  We  are  there- 
fore shown  the  existence  of  two  kinds  of  electricity,  which  are  called 
vitreous  or  POSITIVE  electricity,  and  resinous  or  NEGATIVE  electricity, 
depending  on  whether  they  are  produced  by  rubbing  glass  with  silk, 
or  resinous  materials  with  wool.  The  action  of  the  pith  balls  also 
shews  that  bodies  charged  with  0ne  kind  of  electricity  repel  those 
charged  with  the  same  kind,  but  attract  those  charged  with  the  opposite 
kind.  Charged  bodies  are  also  said  to  be  EXCITED  or  ELECTRIFIED. 

Other  similar  manifestations  of  electricity  may  be  easily  produced. 
For  instance,  if  a  well  dried  sheet  of  paper  be  laid  on  a  table  and 
briskly  rubbed  with  a  rubber  eraser  or  a  coat  sleeve,  it  will  adhere  to 
the  table  and  when  it  is  slowly  raised  by  one  corner,  small  sparks 


may  be  seen  to  pass  between  it  and  the  table.  On  dry  days  it  is 
sometimes  possible  for  a  person  to  gather  a  charge  on  his  body  by 
shuffling  across  the  carpet;  this  charge  may  be  sufficient  to  produce  a 
spark  if  the  finger  be  presented  to  a  gas-fixture  or  to  another  person. 
Again,  if  a  charged  body  be  held  near  to  the  face  a  peculiar  cob- 
webby sensation  may  be  felt  on  account  of  the  attraction  of  the 
small  hairs  of  the  cheeks  by  the  charge. 

If  the  wool  used  to  develop  a  charge  on  the  sealing  wax  by 
rubbing  be  now  tested  by  bringing  near  it  a  charged  pith  ball,  it  will 
also  be  found  to  be  charged — the  charge  being  positive.  In  the 
same  way  the  silk  which  was  used  in  rubbing  the  glass  may  be  found 
to  be  negatively  charged.  This  is  in  accordance  with  a  fact  which  has 
been  experimentally  proved,  that  whenever  a  charge  of  one  kind  is 
developed,  an  equal  charge  of  opposite  kind  is  also  developed.  When 
two  dry  bodies  of  different  materials,  which  do  not  have  the  power 
of  conducting  electricity,  are  rubbed  together,  they  always  become 
charged  with  opposite  kinds  of  electricity.  If  one  of  these  bodies  be 
rubbed  with  a  third  material  its  charge  may  be  changed.  The  kind 
of  charge  which  appears  on  a  body  of  one  material  when  rubbed 
with  another  material  depends  altogether  on  the  nature  of  the  two 
materials.  For  instance,  as  we  have  seen,  when  glass  is  rubbed  with 
silk,  the  glass  becomes  positively  charged  and  the  silk  negatively 
charged.  If  a  stick  of  sulphur  be  rubbed  with  silk  the  order  is 
reversed  and  the  silk  becomes  positively  charged,  while  the  sulphur  is 
negatively  charged.  It  is  possible  to  arrange  a  table  of  materials 
placed  in  such  an  order  that  when  any  two  materials  named  in  the 
table  are  rubbed  together  the  one  that  stands  earliest  in  the  table  will 
ordinarily  become  positively  charged  and  the  other  negatively  charged. 
The  following  table  is  so  arranged.  Its  correctness  may  be  easily 
tested  by  experiments: 

i. — Fur.  7. — Wood. 

2.— Wool. '  &  —Metals. 

3. — Some  resinous  substances.  9.  — Sulphur. 

4. — Glass.  10. — Some  resinous  substances. 

5. — Cotton.  1 1 .  — India  rubber. 

6. — Silk.  12. — Gutta-percha. 

The  reason  for  this  difference  in  materials  is  not  known,  and  in 
fact  slight  differences  in  the  constitution  or  the  surface  of  the  mate- 
rials may  cause  them  to  change  their  relative  positions,  so  that  sim- 
ilar tables  given  in  various  books  do  not  all  agree. 

If  a  piece  of  metal  be  held  in  the  hand  and  rubbed,  no  apparent 
charge  can  be  discovered  on  it.  This  is  because  the  metal  has  the 
power  of  readily  conducting  electricity,  exactly  as  it  has  the  powei 
of  conducting  heat,  and  the  electricity  therefore  all  flows  away  into 
the  body  of  the  operator,  or  through  his  body  into  the  earth.  The 
same  thing  is  true  of  any  of  the  substances  named  in  the  table  if 
they  are  dampened  with  water,  because  water  has  the  power  to  a 
limited  degree  of  conducting  electricity.  Consequently  experiments 


in  statical  electricity  cannot  be  readily  made  on  a  damp  day  or  when 
the  materials  are  damp. 

If  the  metal  be  fastened  in  a  handle  of  dry  wood  or  hard  rubber 
and  again  rubbed,  it  will  become  charged.  This  is  because  the  wood 
or  hard  rubber  does  not  have  the  power  of  conducting  the  electricity 
to  an  extent  which  is  here  appreciable  and  it  therefore  cannot 
escape. 

Materials  which  readily  conduct  electricity  are  called  conductors, 
and  thoss  which  either  do  not  conduct  it  at  all  or  only  conduct  it  in 
a  very  small  degree,  are  called  non-conductors  or  insulators.  An  inter- 
mediate class  which  have  the  conducting  power  to  a  considerable 
degree  are  often  called  partial  conductors. 

The  following  table  gives  a  list  of  materials  placed  approxi- 
mately in  the  order  of  their  conducting  powers: 

1.  Metals.  7.  Various  oils.         13.   Vulcanite. 

2.  Charcoal  and  graphite.    8.  Dry  wood.  14.    Paraffine. 

3.  Acids.  9.  Silk.  15.   Porcelain. 

4.  Salty  solutions.  10.  India  rubber.        16.   Glass. 

5.  Plants  and  animals.       n.  Mica.  17.    Dry  air. 

6.  Pure  water.  12.  Shellac. 

We  ordinarily  restrict  the  term  conductor  to  the  metals.  The 
materials  in  the  table  numbered  from  two  to  six  may  be  called  par- 
tial conductors,  and  the  last  eleven  materials  may  be  called  insula- 
tors. Of  all  the  materials  named  dry  air  may  be  said  to  be  the  only 
one  which  has  absolutely  no  conducting  power  under  ordinary  con- 
ditions, though  that  of  glass,  porcelain,  etc.,  is  exceedingly  small. 

The  cause  of  the  difference  in  the  conducting  power  of  the 
various  materials  is  not  known  and  will  probably  not  be  known 
until  the  exact  constitution  of  electricity  is  determined.  By  means 
of  the  great  conducting  power  or  conductivity  of  metals  electricity 
may  be  conveyed  from  place  to  place.  If,  for  instance,  two  blocks 
of  metal  connected  by  a  wire  be  mounted  on  insulators,  then,  if  a 
charge  be  given  to  one,  part  of  the  electricity  will  flow  along  the 
wire  to  the  second  block,  electrifying  it.  A  conductor  which  is  sup- 
ported on  insulators  in  such  a  way  that  electricity  cannot  escape  from 
it  is  said  to  be  insulated. 

A  body  may  be  charged  or  electrified  by  the  influence  upon  it 
of  a  charged  body.  Thus  suppose  a  brass  ball  be  insulated  and 
charged,  and  then  be  brought  near  an  uncharged  insulated  brass  ball. 
The  second  ball  will  be  found  to  be  charged,  if  it  is  tested  by  bring- 
ing a  charged  pith  ball  near  to  it.  A  charge  which  is  thus  devel- 
oped by  the  influence  of  a  charged  body  on  a  NEUTRAL  or  uncharged 
one,  is  said  to  be  developed  by  INDUCTION.  If  the  brass  ball  on 
which  a  charge  is  thus  INDUCED,  be  carefully  examined,  its  two  sides 
will  be  found  to  hold  opposite  kinds  of  electricity  (Fig.  i).  The 
side  of  the  second  ball  which  is  away  from  the  first  ball  will  hold  the 
same  kind  of  electricity  as  the  latter,  and  the  side  which  is  near  the 
first  ball  will  hold  the  opposite  kind.  This  is  in  accordance  with  the 


law  of  attraction  and  repulsion  between  the  different  kinds  of 
electricity,  given  in  this  lesson.  For  example,  if  the  first  ball  (A 
in  Fig.  i)  be  positively  charged,  the  side  of  the  second  (B  in  Fig. 
i)  which  is  away  from  the  first  will  be  positively  charged  and  the 
near  side  will  be  negatively  charged.  This  is  the  condition  shown 
in  the  figure,  where  the  plus  or  positive  sign,  +,  represents  a  posi- 
tive charge,  and  the  minus  or  negative  sign,  — ,  represents  a  nega- 
tive charge.  Now  if  the  second  ball  be  touched  for  an  instant  when 
it  is  very  close  to  the  first,  the  positive  charge  will  immediately  flow 
away  into  the  operator's  body  on  account  of  the  repulsion  of  the  posi- 
tive charge  which  is  on  the  first  ball.  The  negative  charge  on  the 
second  ball  will  remain  on  account  of  the  attraction  of  the  charge  on 
the  first  ball.  If  the  second  ball  now  be  removed  from  the  influence 
of  the  first  ball  it  will  remain  negatively  charged,  the  charge  spread- 
ing all  over  it.  If  the  two  balls  be  now  brought  into  contact  the 
two  charges  will  combine  and  the  two  balls  will  become  neutral. 
The  latter  shows  that  an  induced  charge  is  equal  in  quantity  to  the  charge 
which  induces  it.  This  fact  is  strictly  true,  but  in  many  cases  the 
induced  charge  is  divided  among  several  bodies  which  are  near  a 
charged  body.  The  induced  charge  is  to  be  found  wholly  on  one 
body  only  when  it  completely  surrounds  the  charged  one,  or  is  very 
much  nearer  it  than  any  other  bodies. 

The  object  of  using  brass  balls  in  such  experiments  is  simply  to 
obtain  convenient  and  inexpensive  conductors.  Any  other  materials 
will  give  similar  results,  but  in  the  case  of  poorly  conducting  bodies 
it  is  more  difficult  to  perceive  the  results  on  account  of  the  difficulty 
presented  to  the  distribution  of  the  electricity  under  the  influence  of 
induction. 

The  means  of  detecting  a  charge  thus  far  mentioned  has  been 
through  the  attraction  or  repulsion  of  charged  pith  balls  or  other 
light  objects.  Various  other  means  may  be  used,  all  of  which  are 
dependent  upon  electric  attractions  arid  repulsions  for  their  indi- 
cations. 

Devices  or  instruments  for  determining  the  presence  of  an  elec- 
tric charge  are  called  ELECTROSCOPES.  The  simplest  electroscope  is 
a  charged  pith  ball.  A  very  sensitive  one  is  made  by  attaching  two 
narrow  strips  of  ordinary  gold  leaf  to  the  end  of  a  brass  rod  and 
hanging  the  leaves  in  a  glass  bottle  to  insulate  them  and  protect 
them  from  injury  (Fig.  2).  If  a  charged  body  be  brought  near  the 
top  of  the  rod  which  is  connected  to  the  gold  leaves,  the  rod  and 
leaves  are  electrified  by  induction.  If  the  charged  body  be  a  rubbed 
glass  rod  which  is  positively  charged,  as  in  Fig.  3,  a  negative  charge 
will  appear  at  the  top  of  the  conductor  and  a  positive  one  in  the 
leaves  (compare  the  case  of  the  brass  balls  given  above).  In  this 
case,  since  the  two  leaves  have  charges  of  the  same  kind,  they  will 
repel  each  other  and  separate  (Fig.  3).  The  gold  leaves  are  so  sen- 
sitive that  they  are  likely  to  be  torn  by  the  force  of  their  repulsion 
if  a  heavily  charged  body  is  brought  too  close. 


FIG.  2. 


FIG.  3. 


If  while  the  glass  rod  is  still  held  near  the  electroscope  the  brass 
rod  of  the  electroscope  be  touched  by  the  hand,  the  positive  charge 
in  the  leaves  will  at  once  flow  off  into  the  operator's  body  on  account 
of  the  repulsion  of  the  charge  on  the  glass,  and  the  leaves  will  drop 
together.  The  negative  charge  will  remain  in  the  electroscope  rod 
on  account  of  the  attraction  of  the  charge  on  the  glass  (compare 
brass  balls  above).  Now  if  the  glass  rod  be  taken  away  the  nega- 
tive charge  will  spread  all  over  the  electroscope  rod  and  gold  leaves c 
and  the  leaves  will  again  separate.  It  can  be  easily  proved  that  the 
charge  on  the  gold  leaves  is  now  negative  by  bringing  a  charged 
glass  rod  near  the  top  of  the  electroscope,  when  the  charge  will  be 
attracted  out  of  the  leaves  and  they  will  fall  together.  Or,  if  a 
negatively  charged  rod  of  sealing  wax  be  brought  near  the  top  of  the 
electroscope,  the  charge  in  the  instrument  will  all  be  repelled  into 
the  leaves  and  they  will  separate  farther. 

With  this  simple  device  it  is  possible  to  detect  very  small  charges 
of  electricity.  The  electroscope  may  of  course  be  directly  charged 
by  contact  with  a  charged  body,  but  the  leaves  are  likely  to  be  torn 
by  the  violence  of  the  action,  unless  the  charge  is  quite  small. 

We  are  now  in  a  position  to  see  the  reason  for  the  attraction 
which  rubbed  amber,  rubbed  glass,  and  other  charged  bodies,  have 
for  light  objects.  Since  electric  induction  acts  between  any  charged 
body  and  any  other  body  which  is  reasonably  near,  the  effect  of  the 
charged  body  on  a  light  object  is  first  to  charge  it  by  induction.  The 
positive  and  negative  charges  induced  on  the  light  object  are  equal 
in  quantity.  One  of  them  is  attracted  and  the  other  is  repelled  by 


o 

B 


FIG.  1.  FIG.  4. 

the  original  charge.  That  which  is  attracted  is  nearest  the  original 
charge,  so  that  the  force  of  attraction  is  greater  than  the  force  of 
repulsion.  The  condition  is  illustrated  in  Fig.  i,  where  a  positive 
charge  is  seen  at  a  on  the  large  ball.  This  induces  the  negative  and 
positive  charges  b  and  c  on  the  small  ball.  Since  b  is  considerably 
nearer  a  than  is  c,  the  attraction  between  a  and  b  is  materially  greater 
than  the  repulsion  between  a  and  c.  The  small  ball  is  therefore 
attracted  towards  the  large  ball.  If  the  balls  come  in  contact  the 
small  ball  receives  a  part  of  the  positive  charge  belonging  to  the 
large  one,  and  they  at  once  separate  on  account  of  tha  repulsion  of 
the  two  positive  charges. 

The  attraction  between  a  charged  body  and  an  uncharged  one, 
or  between  two  charged  ones,  always  exists,  though  the  pull  or  push 
exerted  by  the  charges  is  usually  sufficient  to  move  the  bodies  only 
when  they  are  very  light. 

The  actual  force  of  attraction  or  repulsion  exerted  between  any 
two  bodies  depends  upon  the  product  of  the  quantities  of  electricity 
in  their  charges,  their  distance  apart,  and  the  material  which  is 
between  them.  If  they  are  surrounded  by  air  the  push  or  pull  which 
two  charged  bodies  exert  on  each  other  increases  directlv  with  the 
product  of  the  quantities  of  electricity  which  they  hold,  and  decreases 
directly  with  the  square  af  the  distance  between  the  bodies,  provided 
the  bodies  are  small  compared  with  the  distance  between  them. 

The  unit  quantity  of  electricity  is  called  a  COULOMB,  after  a 
French  experimenter  who  lived  about  the  beginning  of  this  century. 
As  a  rough  analogy  with  the  measurement  of  water  or  gas,  we  may 
say  that  a  coulomb  of  electricity  is  the  equivalent  of  a  gallon  of  water 
or  a  cubic  foot  of  gas. 


The  reason  that  the  force  exerted  between  two  charged  bodies 
depends  on  the  product  of  the  two  quantities  of  electricity,  is  that 
each  coulomb  of  electricity  on  one  body  attracts  or  repels  every 
coulomb  on  the  other  body  with  a  fixed  force,  and  therefore  the  total 
force  of  attraction  or  repulsion  depends  on  the  number  of  coulombs 
on  one  body  multiplied  by  the  number  on  the  other  body. 

Instruments  for  determining  the  quantity  of  electricity  which  is 
held  in  a  charge  on  a  body  by  measuring  its  attraction  for  another 
charged  body,  are  called  ELECTROMETERS.  These  instruments  are 
valuable  for  many  purposes  and  will  receive  more  attention  in  later 
lessons. 

If  the  two  charged  bodies  be  immersed  in  a  liquid  such  as  water 
or  oil,  or  be  separated  by  solids,  the  force  exerted  between  them  is 
decreased.  The  amount  of  the  decrease  depends  upon  the  nature  of 
the  separating  material,  and  is  apparently  due  to  a  difficulty  in  the 
attractive  force  making  its  way  through  the  material. 

The  last  of  the  peculiar  properties  of  electricity  which  we  need 
consider  before  taking  up  the  various  generating  machines,  is  the 
location  on  a  body  which  a  charge  always  takes.  We  often  hear  the 
statement  that  electricity  flows  only  on  the  surface  of  a  wire.  This 
is  entirely  untrue.  When  electricity  flows  or  moves  it  passes  through 
the  substance  of  the  conductor..  In  the  case  of  electricity  at  rest, 
however,  the  case  is  different.  When  electricity  is  at  rest  it  never 
enters  the  substance  of  a  body,  but  stays  strictly  on  the  surface.  It  is 
important  that  this  difference  in  the  action  of  electricity  in  motion, 
or  current  electricity,  and  electricity  at  rest,  or  statical  electricity, 
be  remembered.  Again,  statical  -electricity  not  only  stays  on  the 
surface  of  a  body,  but  it  tends  to  stay  on  the  outside  surface.  Fig. 
4  shows  this  by  the  position  of  the  pith  balls  which  are  suspended 
on  the  inside  and  outside  of  a  hollow  brass  cylinder.  The  cylinder 
being  charged,  the  outside  pith  balls  which  are  in  contact  with  it, 
at  once  diverge  on  account  of  a  charge  which  they  receive  from  the 
cylinder.  This  shows  that  the  outer  surface  of  the  cylinder  is 
charged.  The  inner  pith  balls,  which  are  also  in  contact  with  the 
cylinder,  remain  entirely  inert,  showing  that  there  is  no  charge  on  the 
inner  surface.  This  is  true  whether  the  charge  is  given  to  the 
cylinder  from  the  inside  or  outside,  and  is  to  be  expected  on  account 
of  the  known  repulsion  of  like  charges  or  parts  of  a  charge.  The 
different  parts  of  a  charge  try  to  get  as  far  away  from  each  other  as 
possible,  and  therefore  go  to  the  outer  surface  of  a  body  if  its  conduc- 
tivity is  sufficient  to  permit  it.  A  brass  cylinder  is  here  used  so  that 
the  electricity  may  readily  follow  its  tendency  to  move  to  the  outer 
surface  if  it  be  applied  at  the  inner  surface. 

By  virtue  of  the  fact  that  a  charge  tends  to  stay  on  the  outer 
surface  of  a  body,  it  is  possible  to  entirely  screen  an  object  from  all 
electric  force  by  completely  surrounding  it  with  a  conducting  cage. 
This  is  done  in  making  electrometers,  when  it  is  desirable  to  screen 
the  working  parts  of  the  instruments  from  outside  electric  forces. 

Copyrighted,  1894, 


The  National  School  of  Electricity. 

REVIEW  OF    LESSON  I. 

Points  for    Review.     \.     How    much    is   known   about   the   real   constitution   of 
jiectricity  ? 

2.  What  is  the  origin  of  the  word  electricity  ? 

3.  What  are  the  two  kinds  of  electricity  called,  and  how  is  their  existence  shown  ? 

4.  What  is  the  law  of  attraction  and  repulsion  of    he  two  kinds  of  electricity  ? 

5.  Can  a  charge  of  one  kind  of  electricity  exist  alone  ? 

6.  If  a  body  becomes  positively  charged  upon  rubbing  it  with  some  material,  will  it 
always  become  positively  charged   when   rubbed,  regardless  of  the  material  used  in  rub- 
bing it  ? 

7.  What  precautions  must  be  taken   in  handling  metals  in  order  that  they  may  be 
charged  by  rubbing  ? 

8.  What  are  the  meanings  of  the  terms  electric  conductors,  insulators,  and  electric 
conductivity  ? 

9.  What  materials  are  the  best  conductors,  and  what  the  best  insulators  ? 
10.     How  may  electricity  be  conveyed  from  one  place  to  another  ? 

11.  What  effect  does  a  charged  body  have  on  an  uncharged  conductor  which  is 

brought  near  to  it? 

12.  Why  are  light  bodies  attracted  by  charged  bodies? 

13.  What  is  the  purpose  of  the  electroscope? 

14.  How  can  the  kind  of  electricity  in  a  charge  be  determined  by  using  an  electro- 
scope? 

15.  Upon  what  does  the  force  exerted  between  two  charged  bodies  depend? 

16.  What  is  the  force  equal  to  when  the  charged  bodies  are  surrounded  by  air? 

17.  If  the  bodies  are  surrounded  by  liquids  or  solids  how  is  the  force  affected? 

18.  What  is  the  unit  of  measurement  of  a  quantity  of  electricity  called? 

19.  What  is  the  purpose  of  the  instruments  called  electrometers? 

20.  Upon  what  part  of  a  conductor  does  an  electric  charge  always  remain?     Why? 


II. 

MACHINES  FOR  GENERATING  ELECTRICITY  BY  FRIC- 
TION AND  BY  ELECTRIC  INDUCTION. 

We  will  now  take  up  the  various  machines  for  the  generation 
of  electricity.  From  what  has  preceded,  it  is  evident  that  a  simple 
machine  may  be  made  for  the  generation  of  electricity  by  an 
arrangement  for  continously  rubbing  glass  with  silk  or  other  simi- 
lar material,  with  some  device  added  for  collecting  the  electricity 
which  is  developed.  A  German,  named  Von  Guericke,  first  built 
such  a  machine.  In  this  a  large  ball  of  sulphur  was  revolved. 
When  any  person  pressed  his  dry  hands  upon  the  sulphur  ball  the 
friction  generated  electricity  and  his  body  became  charged.  Later, 
a  glass  cylinder  or  plate  and  a  rubber  of  silk  or  leather  came  into 
use. 

In  such  machines  the  charge  upon  the  glass  is  usually  col- 
lected by  induction.  A  row  of  points,  called  a  comb,  attached  to 
an  insulated  brass  block  is  presented  to  the  charged  surface  of  the 


glass  (Fig.  5).  The  positive  charge  on  the  glass  causes  the  far  side 
of  the  brass  conductor  to  become  positively  charged  and  the  row  of 
points  to  become  negatively  charged.  The  particles  of  air  surround- 
ing the  points  become  negatively  charged  and  are  repelled  off  to  the 
positively  charged  glass.  This  leaves  the  brass  conductor  with  a  posi- 
tive charge,  and  the  negative  charge  of  the  air  particles  neutralizes 
the  positive  charge  of  the  glass,  which  is  therefore  ready  to  be  again 
excited  as  it  again  moves  around  to  the  rubber.  The  action  is  con 
tinuous  while  the  glass  is  revolved. 

By  sprinkling  the  rubber  with  a  conducting  powder  or  511 
amalgam,  made  with  mercury,  the  negative  charge  of  the  rubber 
may  also  be  drawn  away.  If  the  positively  charged  brass  conductor 
is  then  connected  by  a  wire  to  the  rubber,  a  continuous  flow  of  elec- 
tricity will  pass  from  the  brass  conductor  to  the  rubber.  If  there  is 
a*  small  break  in  the  wire  the  electricity  will  jump  across  it  in  the 
form  of  a  spark. 

The  friction  of  a  jet  of  wet  steam  passing  through  a  wooden 
nozzle,  and  many  other  plans,  may  be  used  to  generate  electricity 
in  a  similar  way  by  friction. 

By  the  frictional  methods  the  quantity  of  electricity  generated 
in  a  reasonable  time  is  comparatively  small,  and  machines  operating 
by  induction  may  be  used.  The  simplest  device  of  this  kind  is  called 
an  electrophorus.  This  consists  of  a  plate  of  sulphur,  vulcanized 
rubber,  or  similar  material,  and  a  metal  plate  or  cover  with  an 
insulating  handle  (Fig.  6).  Rubbing  the  sulphur  or  rubber  with 
flannel  electrifies  it  negatively.  When  the  cover  is  set  down  it 
touches  the  base  at  only  a  few  points  on  account  of  its  roughness, 
and  it  becomes  electrified  by  induction  (Fig.  7).  The  negative 
induced  charge  maybe  allowed  to  escape  into  the  operator's  body  by 
touching  the  cover  with  a  finger,  as  explained  in  the  first  lesson. 
The  cover  remains  with  a  positive  charge  which  may  be  used  to 
charge  other  bodies. 

The  process  of  charging  the  cover  maybe  repeated  again  and 
again  without  affecting  the  charge  on  the  base,  but  the  latter  will  be 
slowly  dissipated  through  dampness  in  the  air. 

What  is  known  as  a  Holtz  electric  machine  may  be  roughly 
described  as  an  automatic  electrophorus.  This  consists  of  two  parallel 
plates  of  glass,  one  of  which  is  mounted  to  rotate,  with  certain  induc- 
ing and  collecting  devices  (Fig.  8).  The  following  is  a  brief  expla- 
nation of  the  action  of  this  machine:  at  opposite  points  on  the  sta- 
tionary plate  holes  or  windows  are  cut  and  over  these  are  pasted 
pieces  of  paper  called  sectors.  These  are  given  opposite  charges  by 
means  of  rubbed  rods  of  glass  and  sealing-wax  or  by  other  means. 
In  front  of  the  revolving  plate  opposite  each  sector,  is  a  comb.  The 
charges  on  the  sectors  act  indirectly  on  the  combs  and  the  conductors 
attached  to  them,  so  that  the  knobs  that  terminate  the  conductors 


10 


.  6 


are  charged  with  opposite  kinds  of  electricity.  The  electricity  which 
is  attracted  into  the  combs,  flows  off  onto  the  revolving  plate  exactly 
as  was  explained  in  the  case  of  the  cylinder  friction  machine,  and 
charges  it  as  shown  in  Fig.  9.  The  charges  on  the  revolving 
glass  are  carried  around  under  the  opposite  combs  and  act  inductively 
on  them,  and  are  then  neutralized  by  the  charges  on  the  streams  of 
air  particles  passing  off  the  combs.  If  the  two  knobs  be  placed  in 
connection  a  flow  of  electricity  passes  through  the  conductors  out  of 
the  combs  onto  the  revolving  plate,  which  is  thus  kept  charged  as  in 
Fig.  9,  and  the  current  of  electricity  continues  as  long  as  the 
plate  is  revolved.  If  the  plate  is  revolved  with  sufficient  rapidity  a 
spark  will  jump  from  knob  to  knob,  thus  completing  the  circuit  even 
when  the  knobs  are  a  considerable  distance  apart. 


Fid.  7 


ii 


In  starting  the  machine  it  is  really  sufficient  to  charge  only  one 
of  the  sectors,  as  the  other  will  then  become  charged  through  the 
action  of  the  machine.  It  is  not  necessary  to  go  fully  into  the  action 
of  these  machines  or  into  that  of  various  devices  to  increase  their 
effectiveness  and  make  them  self-exciting. 

The  action  of  these  machines  may  be  compared,  as  a  rough  but 
handy  analogy,  to  pumps  for  circulating  water  or  gas  through  a  sys- 
tem of  pipes.  The  machines  act  as  though  they  were  machines  foi 
pumping  electricity. 

Suppose  a  water  tank  to  be  placed  in  the  basement  and  another 
in  the  garret  of  a  house,  and  a  pump  be  connected  in  the  pipe  lead- 
ing from  one  to  the  other.  If  the  tanks  are  full  of  water  and  the 
pump  be  started,  water  will  be  drawn  from  the  lower  tank  and  sent 
into  the  upper  one,  which  will  overflow,  and  unless  the  water  is 
caught  it  will  run  down  to  the  ground.  If  an  overflow  pipe  be  car- 
ried from  the  upper  tank  to  the  lower  one,  the  overflow  will  run 
back  into  the  lower  tank,  and  the  water  will  be  singly  circulated 
by  the  pump  through  the  system  of  pipes  between  the  two  tanks. 

This  is  similar  to  the  conditions  of  an  electrical  machine  when 
the  positive  and  negative  terminals  are  connected  together  or  sparks 
are  passing  between  them. 

Now,  if  the  overflow  pipe  is  stopped  up  and  drip  pans  be  arranged 
so  that  water  from  the  upper  tank  cannot  run  down  into  the  lower 
one,  the  pump  will  soon  empty  the  lower  tank,  after  which  it  may 
continue  to  run,  but  it  cannot  pump  any  water  and  no  stream  will 
flow  through  the  pipes. 

In  the  same  way  if  the  two  conductors  of  an  electric  machine  are 
not  connected,  and  are  too  far  apart  for  a  spark  to  pass  between 
them,  the  conductors  will  be  strongly  charged  with  opposite  kinds  of 
electricity,  but  then  the  action  of  the  machine  in  circulating 
electricity  will  cease  until  a  path  is  provided  for  the  current  to 
flow. 

The  quantity  of  water  circulated  by  the  pump  depends  upon  the 
pressure  which  it  produces,  and  upon  the  size  of  the  pipes,  and  a  sim- 
ilar rule  holds  for  the  circulation  of  electricity  by  an  electrical 
machine.  The  volume  of  the  stream  of  water  may  be  designated  as 
a  certain  number  of  gallons  or  cubic  feet  per  second.  In  the  same 
way  the  volume  of  a  current  of  electricity  may  be  designated  as  one 
which  conveys  a  certain  number  of  coulombs  per  second.  An 
electric  current  carrying  one  coulomb  per  second  is  called  a  current 
of  one  ampere,  and  the  volume  of  electric  currents  is  always  given  in 
amperes.  This  name  was  given  in  honor  of  a  great  French  scientist 
whose  name  was  Ampere. 

To  pass  a  stream  of  a  certain  number  of  gallons  per  minute 
through  a  certain  pipe,  demands  the  application  of  a  certain  pressure 
to  overcome  the  frictional  resistance.  In  the  same  way  it  requires  a 


certain  electrical  pressure  to  pass  a  given  electrical  current  through 
any  conducting  wire,  on  account  of  the  resistance  which  the  wire 
offers  to  the  flow  of  the  electricity.  The  resistance  to  the  passage  of 
electricity,  or  the  electrical  resistance,  of  any  material,  is  the  recipro- 
cal or  opposite  of  its  conducting  power.  The  greater  its  conducting 
power,  the  less  is  its  electrical  resistance. 

We  usually  speak  of  water  pressure,  or  the  pressure  of  gas,  in 
pounds  per  square  inch,  or  in  feet  difference  of  level,  or  head.  The 
corresponding  unit  of  electrical  pressure  is  a  volt,  which  was  named 
after  Volta,  a  great  Italian  scientist. 

Return  again  to  the  pump  and  tanks.  When  the  pump  is  set 
in  motion  it  sets  up  a  difference  of  pressure  which  may  be  measured 
by  a  gauge,  and  this  starts  the  water  to  flowing  if  it  has  an  outlet.  In 
the  same  way  we  may  look  upon  electrical  machines  as  setting  up  a 
difference  of  electrical  pressure  (which  may  be  measured  by  a  proper 
electrical  instrument),  and  this  starts  the  electricity  to  flowing  if  it 
has  an  outlet. 

This  leads  us  to  the  necessity  of  considering  a  positive  charge 
of  electricity  as  electricity  at  high  electrical  pressure  or  high  potential, 
and  a  negative  charge  as  electricity  at  low  electrical  pressure  or  low 
potential. 

When  a  point  of  high  electrical  pressure  is  connected  by  a  con- 
ducting wire  to  a  point  of  low  pressure,  electricity  will  flow  from  the 
point  of  high  electrical  pressure  to  the  point  of  low  electrical  pres- 
sure until  the  pressure  is  equalized,  unless  the  pressure  is  continually 
kept  up  by  a  machine;  exactly  as  when  two  tanks  standing  side  by 
side  are  filled  with  water  to  different  heights,  if  they  be  connected 
by  a  pipe  water  will  flow  from  one  to  the  other  until  its  level  is  the 
same  in  both. 

In  using  these  comparisons  it  must  be  remembered  that  we  do 
not  touch  upon  the  true  nature  of  electricity,  which  is  unknown,  but 
only  upon  the  laws  of  its  action  which  have  been  experimentally 
determined.  Also,  that  while  water  and  gas  may  be  directly  perceived 
by  our  senses,  electricity  is  absolutely  impalpable — that  is,  it  cannot 
be  perceived  by  the  senses,  and  the  only  way  in  which  we  may 
recognize  it  is  by  its  various  effects. 

Before  leaving  the  question  of  electrical  machines  working  by 
friction  and  induction,  it  is  well  to  call  attention  to  the  great  pres- 
sure of  the  electricity  generated  by  them.  This  is  shown  by  the 
sparks  which  may  be  caused  by  them  to  pass  through  the  air,  or 
even  to  pierce  wood,  glass,  or  other  solid  insulators.  These  effects 
may  be  called  miniature  lightning  effects,  for  lightning  is  simply 
caused  by  the  passage  through  the  air  of  a  current  of  electricity 
under  enormous  pressure.  Thunder  is  like  the  crackle  of  the  spark 
from  an  electrical  machine  greatly  magnified. 


While  the  electrical  pressure  generated  by  these  machines  is 
very  great,  the  quantity  of  electricity  generated  is  quite  small,  and  for 
commercial  purposes  in  which  a  considerable  volume  of  electricity  is 
needed,  other  methods  of  generating  the  current  are  used.  An 'ex- 
planation of  these  will  come  in  the  following  lessons. 

Copyrighted,  1894, 


The  National  School  of  Electricity. 

REVIEW  OF   LESSON  II. 

Points  for  Review.  1.  How  can  a  machine  be  made  to  continuously  generate 
electricity  by  friction? 

2.  How  does  an  electrophorus  work? 

3.  How  can  a  machine  be  made  to  generate  electricity  by  induction? 

4.  What  are  the  units  of  measurements  of  electric  current  and  pressure  called? 

5.  How  much  electricity  is  conveyed  by  a  unit  current  in  each  second? 

6.  If  a  conductor  charged  positively  be  connected  to  one  without  a  charge,  what 
happens? 

7.  If  a  conductor  charged  negatively  be  connected  to  one  without  a  charge,  what 
happens? 

8.v  If  a  conductor  charged  positively  be  connected  to  one  charged  negatively,  what 
happens? 

9.  Why  are  friction  and  induction  machines  not  generally  used  in  general  commer- 
cial service? 

10  What  is  the  relation  of  lightning  to  the  sparks  given  by  an  electrical  machine? 


LESSON  III. 

ELECTRIC  BATTERIES,  OR  APPLIANCES  FOR  GENER- 
ATING ELECTRICITY  BY  CHEMICAL  ACTION. 

One  of  the  effects  of  chemical  action  is  to  give  out  heat.  When 
wood  or  coal  is  burned  the  carbon  of  the  burning  material  combines 
with  oxygen  of  the  air,  and  heat  is  given  out  as  the  result  of  the 
chemical  combination,  which  we  call  combustion.  In  the  same  way 
if  zinc  be  dissolved  in  sulphuric  acid,  the  acid  combines  with  the- 
zinc  and  heat  is  given  off  as  the  result  of  the  chemical  combination. 
This  heat  represents  a  certain  energy  or  capacity  for  doing  work.  It 
has  been  found  that  under  certain  conditions  the  energy  thus  repre- 
sented by  chemical  action  may  be  converted  iuto  an  electric  current, 
and  taking  advantage  of  this  we  get  electric  batteries.  Electric 
currents  produced  by  chemical  action  were  first  observed  and  studied 
about  the  end  of  the  last  century  by  Galvani  and  Volta,  both  of 
whom  were  Italian  scientists.  Volta  will  be  recognized  as  the  man 
from  whose  name  comes  the  word  volt,  the  name  of  the  unit  of 
electrical  pressure. 

When  two  plates  of  different  metals  are  placed  so  that  they  do 
not  touch  each  other  in  a  liquid  (Fig.  10)  which  is  inclined  to  attack 
them  chemically,  one  of  them  becomes  positively  charged  and  the 
other  negativaly  charged  with  electricity.  The  charges  are  so  minute 
that  they  cannot  be  distinguished  by  the  electroscopes  previously 
explained,  but  a  delicate  electrometer  will  distinguish  and  measure 
the  charges.  If  the  metals  be  connected  by  a  wire  a  current  flows 

" 


through  it  from  one  plate  to  the  other,  and  'this  may  be  readily  dis- 
tinguished by  its  effects,  which  will  be  explained  later.  The  positive 
or  high  pressure  plate  is  the  one  which  is  attacked  least  readily  by 
the  liquid. 

A  cup  containing  two  plates  thus  immersed  in  a  liquid  is  called 
an  electric  battery  cell,  and  the  plates  are  called  the  poles  or  electrodes 
of  the  cell.  It  is  usual  to  speak  of  the  pole  which  is  at  the  higher 
pressure,  and  from  which  the  current  flows  through  the  wire,  as  the 
positive  pole;  the  other  pole  is  then  called  the  negative  pole.  The 
difference  of  electrical  pressure  between  the  poles  is  called  the 
electromotive  force  of  the  cell.  The  phrase  electromotive  force  means 
that  which  tends  to  move  electricity,  that  is,  a  difference  of  electrical 
pressure.  This  phrase  is  often  abbreviated  into  E.  M.  F.  We  will 
generally  speak  of  it,  however,  as  the  electrical  pressure  of  the  cell. 
An  electric  battery  is  often  called  a  galvanic  or  voltaic  battery,  and 
the  electricity  produced  by  a  battery  is  often  called  galvanic  or 
voltaic  electricity,  although  the  electricity  is  exactly  the  same  as  that 
produced  in  any  other  way.  These  terms  are  applied  in  the  same 
way  as  the  terms  spring  water  and  well  water,  for  instance,  are 
applied  to  pure  water  which  is  drawn  from  a  spring  or  a  well,  though 
the  water  does  not  differ  from  pure  water  drawn  from  other  sources. 

When  the  poles  of  a  cell  are  connected  by  a  wire  it  is  found  that 
an  electric  current  not  only  flows  from  the  positive  to  the  negative 
pole  through  the  wire,  but  it  continues  through  the  liquid  from  the 
negative  to  the  positive  pole.  If  the  current  is  followed  from  any 
point  in  its  flow,  it  will  be  found  to  return  through  a  complete  path 
to  the  same  point,  exactly  as  water  is  circulated  by  a  pump  through 
a  system  of  pipes.  A  continuous  current  of  electricity  is  therefore 
said  to  flow  in  a  complete  path  or  circuit.  A  complete  circuit  is  often 
called  a  closed  circuit.  The  current  inside  the  cell  then  is  driven, 
by  the  effect  of  chemical  action  against  a  difference  of  pressure,  just 
as  water  is  raised  against  a  difference  of  pressure  by  a  pump.  Out- 
side of  the  cell  where  there  is  no  restraining  action  besides  that  o^. 
the  electric  resistance  of  the  connecting  wire,  the  current  follows  its 
own  tendency  to  flow  from  the  point  of  high  pressure  to  that  of  low 
pressure.  We  have  seen  that  electricity  is  conveyed  in  a  similar 
manner  through  a  complete  circuit  by  the  action  of  friction  and 
induction  machines.  When  two  insulated  conductors  at  different 
electrical  pressures  are  connected  by  a  wire  a  brief  current  flows,  just 
as  a  current  of  water  flows  through  the  pipe  connecting  two  tanks  in 
which  the  water  stands  at  different  levels,  but  the  current  ceases  as 
soon  as  the  pressure  is  equalized.  In  order  that  a  continuous  current 
may  be  produced  a  difference  of  electrical  pressure  must  be  continuously 
supplied  in  a  closed  circuit. 

The  magnitude  and  direction  of  the  difference  of  electrical  pres- 
sure between  the  poles  of  a  battery  cell  depend  upon  the  materials 


16 


IINC  PlfiTt 


FIG  1O. 


CARBON  WtiTC. 


CARBON  PJ.XTE' 
^  />0AO(/3    C(V> 

Z/NC  W.AT& 


-  ZINC 


2&. 


17 


in  the  plates  and  the  nature  of  the  liquid.  For  instance,  if  zinc  and 
copper  be  the  plates  of  a  cell  containing  sulphuric  acid,  the  electrical 
pressure  of  the  cell  is  about  nine-tenths  of  a  volt  and  the  copper 
plate  is  the  positive  pole.  If  two  cells  containing  sulphuric  acid  as 
the  liquid  be  made  using  zinc  and  lead  for  the  plates  of  one  and  lead 
and  copper  for  the  plates  of  the  other,  the  lead  is  the  positive  plate 
in  the  former  and  the  negative  plate  in  the  latter.  The  electric 
pressure  developed  in  each  of  these  would  also  be  less  than  in  the 
case  of  the  zinc-copper  cell.  In  cells  which  are  to  be  obtained  from 
dealers  the  negative  pole  is  nearly  always  of  zinc,  but  the  metal 
composing  the  positive  plate  and  the  composition  of  the  liquid  vary 
greatly.  The  positive  plate  is  generally  made  of  copper,  carbon,  or 
platinum,  and  the  liquids  consist  of  various  acids,  sal-ammoniac, 
caustic  potash,  etc. 

If  a  number  of  cells,  such  as  the  zinc-copper  cells  described 
above,  are  connected  in  a  series  with  the  zinc  pole  of  one  connected 
to  the  copper  pole  of  the  next,  the  zinc  pole  of  this  connected  to  the 
copper  pole  of  the  next,  and  so  on  (Fig.  u),  then  the  total  difference 
of  electrical  pressure  between  the  free  copper  and  zinc  poles  is  equal 
to  the  sum  of  the  pressures  developed  by  all  the  individual  cells. 
When  a  battery  is  thus  connected  up  so  that  the  pressures  developed 
in  the  individual  cells  are  all  added  together  the  cells  are  said  to  be 
connected  in  series. 

The  electrical  pressure  of  a  cell  depends  only  upon  the  nature 
of  the  plates  and  the  liquid,  and  is  entirely  independent  of  the  size  of 
the  plates.  This  can  be  easily  proved  by  making  two  cells  out  of 
tumblers  containing  dilute  sulphuric  acid,  in  one  of  which  are  placed 
narrow  strips  of  copper  and  zinc,  and  in  the  other  are  placed  broad 
strips  of  the  metals.  If  these  are  connected  in  series  with  the  free 
poles  joined  by  a  wire,  a  current  will  flow  as  shown  by  the  vigorous 
chemical  action  which  causes  bubbles  to  gather  on  the  copper  plates. 
If  one  of  the  cells  be  now  reversed,  so  that  the  copper  pole  is  con- 
nected to  the  copper  pole  of  the  other,  no  such  action  will  occur, 
showing  that  the  electrical  pressures  which  tend  to  send  currents  in 
opposite  directions  are  equal  and  neutralize  each  other. 

If  the  two  poles  of  a  zinc-copper  cell,  such  as  we  have  been 
considering,  be  connected  by  a  wire,  a  vigorous  chemical  action  goes 
on  at  first,  but  it  gradually  decreases  in  intensity  and  finally  appears 
to  stop  altogether.  This  effect  may  be  plainly  shown  by  connecting 
an  electric  bell  in  the  circuit  of  the  cell.  When  the  circuit  is  first 
completed  the  bell  will  ring  loudly,  but  it  will  soon  weaken  and 
after  a  time  cease  to  ring  altogether.  If  the  cell  be  then  examined  a 
layer  of  bubbles  will  be  found  upon  the  copper  plate.  These  bubbles 
are  composed  of  hydrogen  gas  which  is  liberated  from  the  sulphuric 
acid  by  the  chemical  action  in  the  cell.  The  effect  of  these  hydro- 
gen bubbles  is  two-fold.  First,  they  tend  to  set  up  a  counter  electric 


18 


pressure  in  the  cell,  or  a  pressure  which  is  opposite  in  direction  to 
that  due  to  the  regular  action  of  the  cell,  and  thus  the  effective  pres- 
sure of  the  cell  is  reduced,  and  second,  the  layer  of  bubbles  presents 
a  hijgh  resistance  to  the  flow  of  the  current.  A  cell  which  is  made 
inactive  by  a  layer  of  hydrogen  bubbles  is  said  to  be  polarised,  and 
the  effect  is  called  polarisation. 

In  order  that  a  cell  may  be  capable  of  working  continuously 
some  plan  must  be  adopted  to  keep  it  from  polarizing,  or,  as  it  is 
often  called,  to  keep  it  depolarised.  This  may  be  effected  in  three 
different  ways:  ist,  by  mechanical  action;  ^  by  direct  chemical 
action,  which  absorbs  the  hydrogen;  3d,  by  electro-chemical  action, 
by  which  the  hydrogen  is  exchanged  "for  a  metal  which  is  deposited 
upon  the  positive  plate. 

The  first  method  of  depolarizing  requires  that  the  hydrogen 
bubbles  be  cleared  off  the  positive  plate  as  fast  as  they  are  deposited 
upon  it.  This  may  be  done  by  continuously  stirring  the  liquid  or 
blowing  air  into  it.  If  the  positive  plate  be  well  roughened  the 
hydrogen  bubbles  will  not  stick  to  it  so  closely,  but  many  will  float 
off  to  the  surface  of  the  liquid  and  escape.  This  plan  was  used  in  a 
cell  commonly  called  Smee's  cell,  which  was  used  commercially 
many  years  ago,  but  it  was  not  very  successful. 

If  some  substance  be  added  to  the  liquid  of  the  cell,  which  will 
combine  with  the  hydrogen  as  quickly  as  it  is  formed  the  polarization 
will  evidently  be  avoided.  This  is  the  foundation  of  the  second 
method  of  depolarizing.  Various  substances  may  be  used  for  this 
purpose,  but  dioxide  of  manganese,  bichromate  of  potash,  chloride  of 
lime  bleaching  powder,  ancT  nitric  acid  are  used  most  commonly. 
The  well-known  bichromate  battery,  which  is  often  used  to  run 
small  motors,  ignite  the  gas  in  gas  engines,  and  for  similar  purposes, 
is  a  zinc-carbon  battery,  with  a  liquid  composed  of  sulphuric  acid, 
in  which  bichromate  of  potash  is  dissolved.  When  this  cell  is  in 
operation  polarization  is  prevented  by  the  immediate  combination  of 
the  hydrogenjwhich  is  liberated  from  the  sulphuric  acid  with  the 
bichromate  offpotash.  Carbon  is  used  for  the  positive  plate  in  this 
cell  because  tiae  bichromate  of  potash  will  attack  and  destroy  copper. 
In  the  bichromate  battery  the  zincs  are  generally  arranged  so  that 
they  may  be  lifted  out  of  the  fluid  when  the  cells  are  not  in  use, 
because  the  fluid  eats  up  zinc  when  the  circuit  of  the  cell  is  open.  / 
From  this  comes  the  name///^^?  battery. 

When  nitric  acid  is  used  as  a  depolarizer  it  cannot  be  allowed 
to  come  in  contact  with  the  zinc,  which  it  attacks  vigorously,  conse- 
quently it  is  confined  in  a  porous  earthenware  cup  within  which  is 
the  positive  pole  of  carbon  or  platinum.  Fig.  12  shows  such  a 
cell  complete,  and  Fig.  1 3  shows  the  same,  thing  in  cross-section. 
The  earth  enware/wtf^  cup  is  sufficient  to  prevent  the  liquids  from 
mixing,  but  after  it  has  become  well  soaked  it  does  not  present  much 


resistance  to  the  passage  of  a  current.  The  cells,  which  are  made  tip 
with  nitric  acid  for  the  depolarizer,  are  only  useful  for  furnishing 
current  for  experimental  purposes  and  for  that  purpose  they  are 
are  much  more  expensive  than  dynamos.  They  have,  therefore, 
practically  gone  out  of  use.  The  commonest  forms  of  cells  of  this 
type  are  those  known  as  Bunsen's  and  Grove's  cells.  The  action  of 
nitric  acid  as  a  depolarizer  is  quite  similar  to  that  of  bichromate  of 
potash,  though  it  is  more  powerful. 

When  dioxide  of  manganese  is  used  as  a  depolarizer  it  is  gener- 
ally broken  up  into  small  lumps  and  put  into  a  porous  cup  surround- 
ing a  positive  plate  of  carbon.  When  sal-ammoniac  dissolved  in 
water  is  used  as  the  liquid  in  this  form  of  cell,  it  makes  the  familiar 
Leclanche  battery  (Fig.  14),  which  is  used  so  frequently  for  ringing 
door  bells  and  in  similar  service.  Sometimes  the  dioxide  of  man- 
ganese is  pulverized  and  mixed  with  shellac,  after  which  it  is 
pressed  into  small  bricks,  which  are  placed  upon  either  side  of  the 
carbon  positive  plate  (Fig.  15),  as  in  the  "prism"  Leclanche  battery. 
/The  depolarizing  effect  of  dioxide  of  manganese  is  not  sufficiently 
,  powerful  to  prevent  a  cell  from  becoming  polarized  if  used  con- 
'  stantly.  Consequently  Leclanche  cells  are  only  satisfactory  in  service 
which  is  intermittent  like  ringing  door  bells,  where  the  circuit  is 
open  a  considerable  part  of  the  time  and  the  battery  rests  without 
chemical  action.  Leclanche  cells  are  called  open  circuit  cells  on 
account  of  the  small  chemical  action  which  goes  on  in  them  when 
the  circuit  is  open  and  because  they  are  not  satisfactory  in  continuous 
service. 

Copyright  1894, 


The  National  School  of  Electricity. 

REVIEW  OF   LESSON  III. 

Points  for  Review. — 1.   Upon  what  action  do  electric  batteries  depend  for  their 
operation? 

2.  What  is  electric  pressure  often  called? 

3.  Why   fs  electricity  which  is  generated  by  electric  batteries   called    galvanic   or 
voltaic  electricity? 

4.  Does  it  differ  from  electricity  generated  by  other  means? 

5.  In  what  direction  does  the  electric  current  flow  through  a  battery  cell?     What  is 
its  direction  in  the  circuit  outside  of  the  cell? 

6.  What  is  the  path  through  which  electricity  flows  called? 

7.  How  may  a  continuous  current  of  electricity  be  produced? 

8.  If  a  number  of  battery  cells  be  connected  in  series,  what  is  the  total  pressure 
generated? 

9.  Upon  what  does  the  pressure  produced  by  any  cell  depend? 

10.  What  is  polarization? 

11.  How  may  polarization  be  avoided? 

12.  How  is  a  Leclanche  cell  made  up? 

13.  Why  are  Leclanche  cells  called  open  circuit  cells? 


IV. 

ELECTRIC  BATTERIES,  OR  APPLIANCES  FOR  GENER- 
ATING ELECTRICITY  BY  CHEMICAL  ACTION. 

(CONCLUDED.) 

The  third  method  of  depolarizing  introduces  more  complicated 
chemical  reactions,  but  which  we  need  not  go  into  in  much  detail. 
Through  the  use  of  this  method  cells  are  constructed  which  give 
excellent  results  in  continuous  service,  and  which  are,  therefore, 
called  clo$&d  'circuit  cells.  One  of  these  is  probably  the  most  com- 
monly used  battery  of  any  form.  This  is  the  ordinary  gravity  battery, 
or  copper  sulphate  battery  which  is  used  so  much  in  telegraphy. 

The  original  form  of  cell  from  which  the  gravity  cell  came  is 
one  in  which  the  active  liquid  is  sulphuric  acid,  in  which  is 
immersed  the  zinc  or  negative  plate.  The  copper  plate  is  immersed 
in  a  solution  of  ordinary  copper  sulphate,  or  blue  vitriol  (sometimes 
called  blue  stone).  The  two  solutions  are  separated  by  a  porous  cup. 
In  general  terms  the  chemical  action  which  occurs  when  the  battery 
is  in  action  is  as  follows:  The  sulphuric  acid  attacks  the  zinc,  and 
sulphate  of  zinc  is  formed.  At  the  same  time  hydrogen  is  liberated 
from  the  sulphuric  acid  and  goes  towards  the  copper  plate,  where  it 
would  be  deposited  if  it  were  not  for  the  copper  sulphate  which  sur- 
rounds the  copper  plate.  When  the  hydrogen  gets  into  the  copper 
sulphate  solution,  it  goes  into  combination  and  copper  is  separated 
from  the  solution  and  deposited  upon  the  copper  plate,  which  is 
therefore  kept  bright  and  in  good  working  condition. 


21 


During  the  operation  of  the  cell  the  chemical  action  which  has 
been  briefly  explained  causes  a  change  in  the  character  of  the  solu- 
tions. The  sulphuric  acid  changes  to  a  solution  of  sulphate  of  zinc, 
and  the  copper  sulphate  changes  to  sulphuric  acid.  If  the  sulphuric 
acid  is  replaced  by  a  dilute  or  weak  solution  of  zinc  sulphate,  a  cur- 
rent is  set  up,  as  before,  and  the  chemical  action  is  similar,  but  the 
copper  sulphate  is  converted  into  zinc  sulphate.  In  order  that  the 
depolarizing  action  may  continue  during  the  life  of  the  cell  the 
strength  of  the  copper  sulphate  solution  must  be  kept  up.  This  is 
done  by  putting  crystals  of  copper  sulphate  or  blue  vitriol  in  the  cell 
so  that  they  may  be  dissolved.  Fig.  16  shows  a  cell  of  this  battery 
in  its  original  form,  in  which  it  is  called  DanielPs  battery.  In  the 
figure  the  zinc  plate  is  shown  within  the  porous  cup  at  the  right 
hand  of  the  battery  jar,  and  the  copper  plate  is  at  the  left  hand  of 
the  jar.  Alongside  of  the  copper  plate  is  a  perforated  copper  cage 
in-  which  may  be  put  the  copper  sulphate  for  renewing  the  solution. 

The  suphuric  acid  or  zinc  sulphate  solution  of  this  cell  is  ordi- 
narily much  diluted  or  weakened  by  water,  while  the  copper  sul- 
phate solution  is  kept  quite  strong  or  saturated.  When  in  this  con- 
dition the  solution  of  zinc  sulphatej&lighter  than  the  other  and  will 
float  upon  it,  just  as  oil  floats  on  water.  Consequently  if  the  copper 
surrounded  by  the  solution  of  copper  sulphate  be  placed  in  the 
bottom  of  a  battery  jar,  a  weak  solution  of  zinc  sulphate  or  sulphuric 
acid  may  be  carefully  poured  on  top,  and  the  solutions  will  only  mix 
very  slowly.  The  zinc  may  be  hung  from  the  top  of  the  jar  in  the 
upper  solution  (Fig.  1 7).  This  constitutes  the  gravity  battery,  so- 
called  because  the  solutions  are  separated  by  gravity  through  the 
difference  in  their  densities,  instead  of  by  a  porous  cup. 

In  setting  up  such  a  cell  it  is  usual  to  put  the  copper  in  the 
bottom  of  the  jar  surrounded  by  crystals  of  copper  sulphate.  The  jar  is 
then  filled  with  water  to  near  its  top  and  the  zinc  is  immersed  in  the 
upper  part  of  the  liquid.  The  cell  may  be  placed  on  short  circuit  for  a 
time  and  it  will  work  itself  into  good  operating  condition,  or  a  little 
sulphuric  acid  or  zinc  sulphate  solution  may  be  carefully  poured  into 
the  water  and  the  cell  will  at  once  be  in  condition. 

If  a  gravity  cell  be  allowed  to  stand  upon  open  circuit  the  two 
solutions  will  slowly  mix  by  diffusion.  When  any  of  the  copper 
sulphate  solution  reaches  the  zinc  a  black  deposit  of  oxide  of  copper 
is  made  on  it.  This  puts  the  cell  in  such  condition  that  it  will  not  work 
satisfactorily  until  the  zinc  has  been  cleaned.  When  the  cell  is  in 
operation  the  copper  sulphate  is  changed  into  zinc  sulphate  so 
rapidly  that  it  gets  no  chance  to  mix  with  the  latter.  A  gravity 
battery,  therefore,  is  only  satisfactory  in  a  service  which  keeps  it  con- 
stantly working. 

There  are  various  other  types  of  batteries  in  which  the  third 
method  of  depolarizing  is  used,  but  which  are  not  in  sufficiently  gen- 
eral use  to  make  their  description  desirable  here. 


22 


FIG.  16. 


FIG.  17. 


FIG.  18. 


FIG.  19. 


23 


In  nearly  all  battery  cells  some  chemical  action  by  which  the 
zinc  is  wasted  goes  on  when  the  circuit  is  open.  This  may  also  pro- 
ceed while  the  circuit  is  closed  without  adding  to  the  useful  current 
of  the  cell.  Such  wasteful  chemical  action  is  called  local  action.  It 
is  usually  caused  by  metallic  impurities  in  the  zinc,  which  form  with 
the  zinc  little  electric  batteries  by  the  action  of  which  the  zinc  is 
worn  away  in  spots.  A  similar  action  is  also  caused  in  some  cells  by 
differences  in  the  density  of  the  liquid  at  various  parts  of  the  cell.  In 
this  case  the  zinc  near  the  top  of  the  liquid  is  ordinarily  wasted 
away,  and  may  be  entirely  eaten  off. 

To  avoid  local  action  the  zinc  may  be  amalgamated,  that  is,  its 
surface  may  be  alloyed  with  mercury.  For  this  purpose  the  zinc  is 
cleaned  by  dipping  into  a  dilute  acid  solution  and  it  is  then  rubbed 
with  mercury,  which  makes  a  pasty  alloy  on  the  surface.  The 
Aunties  in  the  zinc  do  not  readily  form  an  amalgam  with  mercury 
arid  are  therefore  covered  up,  while  pure  zinc  is  brought  to  the  sur- 
face. Zinc  for  battery  plates  is  also  sometimes  cast  with  a  small 
percentage  of  mercury  in  its  composition. 

The  amount  of  metal  usefully  consumed  in  a  cell  depends 
directly  upon  the  number  of  coulombs  of  electricity  which  are  per- 
mitted to  pass  through  it.  The  amount  of  hydrogen  gas,  copper, 
or  other  metals  liberated  from  the  liquids  also  depends  upon  the 
number  of  coulombs  of  electricity  which  are  passed  through  the  cell. 
This  may  be  stated  as  a  general  law  of  electro- chemical  action,  that 
the  amount  of  chemical  action  in  a  cell  depends  directly  upon  the 
amount  of  electricity  which  passes  through  it,  and  therefore  the  chemi- 
cal action  is  the  same  in  all  cells  of  a  number  connected  in  series  since 
the  same  amount  of  current  will  flow  through  them  all. 

The  weight  of  a  metal  in  grammes  (metric  measure)  which  is 
dissolved  or  deposited  when  one  coulomb  of  electricity  passes  through 
a  cell,  is  called  the  electrochemical  equivalent  of  the  metal. 

Electric  batteries  in  which  a  metal  is  directly  consumed  by 
chemical  action  for  the  generation  of  an  electric  current,  are  called 
primary  batteries.  In  nearly  all  primary  batteries  the  metal  which 
is  consumed  is  zinc.  The  law  of  electrochemical  action  already 
stated  shows  that  no  current  can  be  produced  without  an  equivalent 
consumption  of  metal,  just  as  an  appreciable  amount  of  heat  cannot 
be  given  out  from  a  fire  without  an  appreciable  consumption  of  coal 
or  Tfrood.  The  consumption  of  zinc  in  a  battery  to  furnish  electrical 
energy  in  the  form  of  an  electric  current  is  similar  to  the  burning  of 
coal  under  a  boiler  to  furnish  steam  power.  It  can  be  readily  seen 
that  zinc  makes  an  expensive  iuel,  though  the  consumption  of  a 
pound  of  zinc  in  a  battery  produces  several  times  as  much  energy  as 
is  produced  by  the  combustion  of  a  pound  of  coal  in  the  furnace  of  a 
boiler,  so  that  batteries  in  which  zinc  is  consumed  cannot  be  used 
commercially  to  furnish  electricity  where  currents  of  great  magni- 


tude  are  required,  as  in  electric  lighting.  For  such  purposes  the 
battery  can  never  compete  with  the  dynamo  driven  by  a  steam 
engine,  unless  a  cell  be  invented  in  which  coal  may  be  economically 
consumed  in  the  place  of  zinc,  and  the  heat  due  to  its  combustion 
be  thus  directly  transferred  into  electrical  energy.  If  this  is  ever 
done  the  electric  battery  will  displace  the  steam  engine,  but  batteries 
in  which  zinc  is  consumed  can  never  economically  furnish  current  for 
light  and  power. 

In  many  domestic  operations,  such  as  ringing  electric  bells, 
regulating  dampers,  etc.,  primary  batteries  hold  an  important  place. 
In  telegraphy  and  telephony,  and  other  commercial  applications  on 
a  large  scale  in  which  a  comparatively  weak  current  is  required, 
they  are  used  in  great  numbers.  They  are  also  used  in  electro-thera- 
peutics and  similar  applications.  For  many  domestic  purposes  the 
work  required  of  a  battery  is  intermittent  and  so  small  that  a 
constant  electromotive-force  cell  is  not  required.  Consequently  many 
batteries  are  made  of  simple  zinc-carbon  cells  in  which  the  liquid  is 
a  solution  of  sal-ammoniac.  These  cells  are  just  like  L,eclanche  cells 
without  the  dioxide  of  manganese  depolarizer.  The  carbon  plate 
is  generally  made  with  a  large  surface  so  that  the  polarization  is  not 
very  rapid. 

If  a  gravity  cell  be  worked  until  its  zinc  is  nearly  used  up  and  a 
current  be  then  passed  through  it  from  the  copper  plate  to  the  zinc- 
plate,  metallic  zinc  will  be  deposited  on  the  zinc  plate  by  the  chemi- 
cal action  due  to  the  current.  The  current  which  separates  the  zinc 
from  the  liquid  is  passed  through  the  cell  against  the  electric  pres- 
sure naturally  developed  by  the  cell,  and  energy  must  be  expended  in 
order  that  the  current  may  flow.  This  energy  is  stored  up  during 
the  process  in  the  deposited  zinc,  and  may  be  returned  when  the  zinc 
is  again  dissolved  through  the  operation  of  the  battery  in  the  ordinary 
manner. 

Alternate  discharging  of  the  battery  by  taking  current,  and 
consequently  energy,  from  it  through  the  consumption  of  zinc,  and 
then  again  charging  it  by  expending  energy  in  the  cell  by  sending 
current  into  it  and  depositing  zinc,  may  be  kept  up  indefinitely. 
Each  time  the  cell  will  give  out  nearly  as  much  energy  due  to  the 
consumption  of  its  zinc  as  was  given  to  it  in  depositing  the  same 
amount  of  zinc. 

A  battery  in  which  energy  may  be  stored  through  the  forced 
chemical  action  called  charging  and  from  which  this  energy  may  be 
then  withdrawn  through  the  natural  action  of  the  cell,  is  called  a 
storage  battery.  Storage  batteries  are  also  called  accumulators  or 
secondary  batteries. 

Commercial  storage  cells  are  usually  made  with  lead  plates  im- 
mersed in  dilute  sulphuric  acid. 


The  chemical  action  which  goes  on  in  these  during  charging 
and  discharging  roughly  consists  in  transferring  oxygen  which 
exists  in  oxide  of  lead  on  the  plates  from  one  plate  to  the  other.  It 
is  desirable  that  the  plates  be  capable  of  holding  a  large  amount  of 
oxide  of  lead  in  order  that  the  cells  may  be  of  large  capacity,  and  they 
are  therefore  made  with  corrugations  or  perforations  in  which  the 
oxide  may  be  fixed.  The  perforated  plates  are  called  grids. 

Sometimes  the  plates  are  made  up  for  use  by  filling  the  perfora- 
tions in  the  plates  with  a  paste  consisting  of  lead  oxide  moistened 
with  sulphuric  acid.  This  process  is  called  pasting,  and  plates  made 
up  thus  are  often  called  pasted  plates,  or  Faure  plates  after  the  name 
of  the  inventor  of  the  method.  Sometimes  the  oxide  is  formed  by 
frequent  charging  and  discharging  of  the  cell.  This  process  is  called 
forming,  and  plates  of  this  kind  are  called  PI  ante  plates,  after  the 
original  inventor  of  the  lead  plate  storage  battery,  who  used  this 
method. 

Figure  1 8  shows  a  lead  plate  storage  cell  in  a  glass  jar,  and  fig- 
ure 19  shows  one  in  a  wooden  box  lined  with  rubber.  In  order  that  the 
cell  may  have  a  capacity  for  a  large  current,  a  number  of  positive  and 
negative  plates  are  put  alternately  in  one  jar  and  are  connected  in 
parallel — that  is,  the  plates  are  connected  so  that  the  current  capacity 
of  the  cell  is  equal  to  the  sum  of  the  capacities  of  the  various  plates, 
but  the  pressure  of  the  cell  is  the  same  as  that  of  a  cell  made  up  of  a 
single  pair  of  plates. 

The  positive  plates  of  a  lead  plate  storage  battery  usually  have  a 
brownish  color  and  the  negative  plates  a  greyish  color.  The  electri- 
cal pressure  produced  by  a  lead  plate  cell  generally  varies  between 
1.8  and  2.3  volts  at  different  conditions  of  the  charge. 

Commercial  storage  batteries  are  made  with  other  liquids  than 
sulphuric  acid  and  other  than  lead  plates,  but  they  cannot  be  given 
consideration  here  as  they  have  not  come  into  common  use. 

Copyrighted,  1894, 


The  National  School  of  Electricity. 

REVIEW  OF   LESSON  IV. 

Points  for  Review.     1.     For  what  kind  of  service  are  closed  circuit  cells  adapted? 

2.  What  is  local  action? 

3.  How  may  local  action  be  avoided? 

4.  What  is  the  law  of  electrochemical  action? 

5.  What  is  the  electrochemical  equivalent  of  a  metal? 

6.  Why  are  primary  batteries  of  the  ordinary  commercial  forms  unable  to  compete 
with  dynamos  driven  by  steam  engines   in  furnishing  electric  currents  for  electric  light 
and  power? 

7.  For  what  purposes  are  primary  batteries  particularly  adapted? 

8.  What  is  the  difference  between  a  primary  battery  and  a  storage  battery? 

9.  Of  what  are  the  common  commercial  storage  batteries  made? 

10.     Why  are  a  large  number  of  plates  connected  together  in  parallel  in  the  usual 
storage  cells? 

LESSON    V. 

THE  NATURE  AND  PROPERTIES  OF  MAGNETISM. 
MAGNETIC  FIELDS. 

The  true  nature  of  magnetism  seems  to  be  very  closely  con- 
nected with  that  of  electricity  and  it  will  probably  not  be  exactly 
known  till  the  exact  nature  of  electricity  is  determined.  The  word 
magnet  probably  comes  from  the  Greek  word  for  the  country  of  Mag- 
nesia, which  is  a  small  division  of  Ancient  Greece,  where  a  deposit 
of  magnetic  iron  ore  or  lodestones  (also  called  loadstones)  was  known 
to  the  Greeks.  Some  of  the  properties  of  magnets  were  known  many 
centuries  before  the  Christian  era.  It  is  said  that  the  Chinese  used 
a  device  similar  to  the  compass  to  guide  their  way  across  the  plains 
of  Tartary  as  early  as  1,000  B.  C.,  and  some  say  much  earlier,  but  in 
Europe  the  use  of  the  compass  did  not  become  general  until  the 
thirteenth  century  of  the  Christian  era.  The  attractive  power  which 
magnets  have  for  iron  is  mentioned  by  many  early  writers — Plato, 
Euripides,  and  Thales,  the  Greek  philosopher  mentioned  in  Lesson  i 
(page  i),  all  speak  of  the  lodestone  or  magnet.  Dr.  Gilbert,  who 
coined  the  word  electrical  (Lesson  i,  page  i),  made  a  great  many" 
experiments  with  magnets  and  magnetic  materials.  Dr.  Gilbert 
seems  to  have  been  the  first  to  notice  that  the  attractive  power  of 
magnets  is  greatest  near  certain  points,  or  poles  as  he  named  them. 

Pieces  of  iron  ore  composed  of  oxide  of  iron,  which  is  called 
magnetite  or  magnetic  iron  ore,  when  in  a  pure  form,  sometimes 
have  the  peculiar  property  of  attracting  pieces  of  iron  and  they  are 
theri  called  lodestones.  The  property  held  by  the  lodestone  is  called 
magnetism,  and  the  body  having  the  property  of  magnetism  is  called 
a  magnet.  The  action  of  magnets  led  some  of  the  earlier  experi- 
menters to  look  upon  magnetism  as  due  to  a  magnetic  fluid,  but  this 
Idea  has  been  proved  to  be  wrong.  It  is  found  that  pieces  of  steel 

27 


FIG.  20.  FIG.  21.  FIG.  23. 

which  touch  a  lodestone  or  other  magnet,  become  magnets.  Magnets 
thus  made  are  sometimes  called  artificial  magnets,  and  lodestones  are 
called  natural  magnets.  When  pieces  of  soft  iron  touch  a  magnet 
they  also  become  magnets,  or  are  magnetized  while  in  contact  with 
the  magnet,  but  when  separated  from  the  magnet  the  magnetism  of 
the  soft  iron  disappears.  This  is  called  temporary  magnetism,  while 
the  magnetism  of  hard  steel  which  remains  permanently  is  called 
permanent  magnetism. 

If  a  magnet  be  suspended  on  a  pivot  or  a  thread  it  wrill  be  found 
to  point  nearly  north  and  south,  and  if  it  is  pivoted  at  the  center  the 
north  end  will  dip  down  as  though  it  were  heavier  than  the  south 
end.  A  small,  elongated  magnet  thus  suspended  is  called  a  magnetic 
needle  (Fig.  20).  If  a  magnetic  needle  be  turned  from  the  direction 
which  it  naturally  takes  when  free  to  swing  horizontally  on  its  pivot, 
it  will  at  once  return,  swinging  to  and  fro  until  it  settles  down  in  its 
original  position.  The  pole  of  a  suspended  needle  which  points  to 
the  north  is  called  the  north  pole  and  the  other  pole  is  called  the 
south  pole.  This  tendency  of  a  magnetic  needle  to  set  itself  north 
and  south  is  the  foundation  of  the  compass,  which  essentially  con- 
sists of  a  magnetic  needle  mounted  over  a  dial.  It  is  usual  in  com- 
passes to  counter-balance  the  needle,  or  pivot  it  so  that  it  will  hang 
horizontally,  but  dip  needles  are  sometimes  constructed  of  magnetic 
\ieedles  mounted  on  horizontal -pivots  (Fig.  21).  When  a  dip  needle 
is  turned  north  and  south  its  north  pole  turns  down  towards  the  earth 
as  already  explained. 

If  a  pole  of  a  magnet  be  brought  near  a  magnetic  needle  it  will 
be  fcnmd  to  attract  one  pole  of  the  needle  and  repel  the  other  pole. 
The  north  pole  of  the  magnet  may  be  determined  by  noting  the  way 
it  stands  when  suspended  by  a  thread,  and  it  will  be  found  that  its 
north  pole  always  repels  the  north  pole  of  the  needle  and  attracts  the 
south  pole  of  the  needle.  The  south  pole  of  the  magnet  acts  in  exactly 
an  opposite  manner.  This  action  shows  that  there  are  tivo  kinds  of 
magnetic  poles  and  that  poles  of  the  same  kind  repel  each  other  and 
poles  of  opposite  kinds  attract  each  other.  This  is  quite  similar  to  the 


FIG.  22. 


FIG.  24.  FIG.  25. 

law  of  the  attractions  and  repulsions  of  electric  charges  given  on 
page  3  of  Lesson  i . 

Magnets  made  from  straight  bars  of  steel  are  called  bar  magnets 
(Fig.  22),  and  those  made  from  bars  of  steel  bent  into  horseshoe  form 
are  called  horseshoe  magnets  (Fig.  23).  The  north  pole  of  a  magnet 
is  often  called  the  positive  or  plus  (-J-)  pole,  and  the  south  pole  is  often 
called  the  negative  or  minus  ( — )  pole.  Since  the  positive  pole  turns 
towards  the  north  it  is  sometimes  called  the  north  seeking  pole,  and 
the  negative  pole  is  in  the  same  way  sometimes  called  the  south 
seeking  pole. 

If  the  experiment  with  a  magnetic  needle,  described  in  the  para- 
graph above,  be  repeated,  but  a  bar  of  soft  iron  be  used  in  the  place 
of  the  magnet,  it  is  found  that  either  end  of  the  iron  bar  attracts 
either  pole  of  the  needle.  If  the  iron  bar  be  laid  with  one  end 
near  the  pole  of  a  magnet  it  may  be  shown  to  be  magnetized  by  mov- 
ing a  magnetic  needle  around  it.  The  needle  will  show  by  its  action 
that  the  end  of  the  iron  bar  which  is  near  the  magnet  pole  has  be- 
come a  pole  of  sign  opposite  to  that  of  the  magnet,  and  the  farther 
end  of  the  bar  has  become  a  pole  of  the  same  sign  as  that  of  the  mag- 
net. The  bar  is  said  to  be  magnetized  by  induction.  The  magnet- 
ism in  the  iron  bar  becomes  stronger  as  it  is  brought  closer  to  the 
magnet  pole,  and  is  greatest  when  the  iron  is  in  contact  with  the 
magnet  pole. 

We  are  now  in  a  position  to  see  why  a  magnet  attracts  a  piece 
of  iron,  and  the  cause'  for  the  effect  of  the  iron  bar  on  the  magnetic 
needle.  When  a  steel  magnet  pole  is  brought  near  to  a  piece  of  iron, 
the  iron  is  magnetized  by  induction.  The  positive  and  negative 
poles  induced  in  the  iron  are  of  equal  magnitude.  One  of  the  induced 
poles  is  attracted  and  the  other  is  repelled  by  the  steel  magnet  pole, 
but  that  which  is  attracted  is  nearest  the  original  magnet  pole  and 
the  force  of  attraction  is  therefore  greater  than  the  force  of  repulsion. 
The  effect  of  a  bar  of  iron  on  a  magnetic  needle  is  caused  in  the  same 
way  by  the  magnetism  induced  in  the  bar  by  the  poles  of  the 
needle. 

The  magnetism  induced  in  a  bar  of  iron  may  induce  magnetism 
in  another  piece,  and  this  in  another  piece,  and  so  on,  but  the  mag- 

29 


netism  in  each  successive  piece  is  weaker  than  in  the  preceding 
piece.  Thus  a  magnet  may  be  made  to  support  a  string  of  several 
nails  end  to  end  (Fig.  24). 

For  every  pole  induced  in  a  piece  of  iron  or  steel  another  pole  of 
equal  strength  is  produced.  For  instance,  if  the  north  pole  of  a  mag- 
net be  touched  to  one  end  of  a  bar  of  iron  a  south  pole  is  induced  in 
that  end,  and  an  equal  north  pole  in  the  other  end.  If  the  two  ends 
of  the  iron  bar  be  touched  by  the  north  poles  of  two  equal  magnets, 
south  poles  are  induced  in  both  ends  of  the  bar.  In  this  case  an 
examination  of  the  bar  with  a  magnetic  needle  shows  that  a  north 
pole,  which  is  equivalent  to  two  poles,  is  produced  near  the  center 
of  the  bar.  Again,  if  a  magnet  be  broken,  it  will  be  found  that 
each  piece  has  two  equal  and  opposite  poles.  We  are,  therefore, 
justified  in  saying  that  for  every  magnet  pole  that  exists,  there  exists 
in  the  same  magnetic  body  an  equal  and  opposite  pole.  This  is  quite 
similar  to  the  existence  along  with  every  electric  charge  of  an  equal 
and  opposite  charge  (Lesson  i,  page  3).  Magnetic  force  will  act 
through  a  vacuum  and  through  all  materials  except  those  in  which 
magnetism  may  be  induced. 

The  actual  force  exerted  between  two  magnets  depends  upon 
the  strength  of  their  poles  and  their  distance  apart.  If  it  were  pos- 
sible to  have  two  separate  magnet  poles  of  small  size  as  compared 
with  their  distance  apart,  the  force  exerted  between  them  would  be 
equal  to  the  product  of  the  strength  of  the  poles  divided  by  the 
square  of  their  distance  apart.  This  is  similar  to  the  law  of  the 
force  exerted  between  two  small  isolated  bodies  holding  electric 
charges. 

The  condition  required  for  this  law  of  force  to  be  fulfilled  can 
only  be  gained  by  using  poles  of  two  very  long,  thin  magnets.  The 
force  between  two  magnets,  which  may  usually  be  measured,  does 
not  follow  this  law  directly,  because  the  poles  are  of  considerable 
magnitude  as  compared  with  their  distance  apart.  Every  small  por- 
tion of  the  pole  of  one  magnet  exerts  a  force  on  every  small  portion 
of  the  pole  of  the  other  magnet  according  to  the  law,  and  when  all 
these  small  forces  are  added  together  the  law  is  apparently  changedv 
though  it  is  based  on  the  fundamental  one. 

Material  in  which  magnetism  may  be  induced,  and  which  is 
therefore  attracted  by  a  magnet,  is  called  magnetic  material.  Iron 
in  its  various  forms  is  the ,  most  strongly  magnetic  material  known. 
There  are  only  a  few  other  materials  that  are  known  to  be  magnetic. 
Of  these  the  metals  nickel  and  cobalt  are  the  commonest.  All  mate- 
rials which  are  not  quite  strongly  magnetic  are  usually  spoken  of  as 
nonmagnetic^  since  they  are  nearly  neutral  as  regards  magnetism. 
Magnetic  materials  are  sometimes  za\\£&  paramagnetic,  and  nonma- 
gnetic materials  are  sometimes  called  diamagnetic. 

As  was  stated  earlier  in  this  lesson,  if  a  magnet  be  broken  each 
piece  will  be  a  complete  magnet,  however  small  the  pieces  may  be. 
This  points  to  the  fact,  which  is  now  generally  believed  to  be  true, 

30 


that  the  final  particles,  or  molecules,  of  magnetic  material  are  little 
magnets,  each  having  its  own  north  and  south  poles.  When  mag- 
netic material  is  unmagnetized  it  is  supposed  that  the  molecules  are 
arranged  in  a  hap-hazard  manner,  or  in  groups,  so  that  they  neutralize 
each  other's  magnetic  effects.  When  the  material  is  subjected  to  the 
influence  of  magnetic  force  the  molecular  magnets  are  all  attracted 
around,  so  that  their  poles  point  more  or  less  in  the  same  direction. 
In  Fig.  25  the  small  blocks  may  be  taken  to  roughly  represent 
magnetic  molecules  which  are  very  greatly  magnified.  The  dark 
ends  represent  their  south  poles  and  the  light  ends  their  north  poles. 
When  they  are  arranged  with  their  like  poles  all  pointing  in  the 
same  direction  as  in  the  figure,  it  is  readily  seen  that  the  poles  in 
the  interior  of  the  material  neutralize  each  other's  effect  and  mag- 
netism shows  at  the  ends. 

It  is  found  that  some  materials  are  more  readily  magnetized  than 
others.  Thus,  soft  iron  is  very  readily  magnetized,  but  loses  almost 
all  of  its  magnetism  if  it  is  slightly  jarred  when  the  external  magnet- 
izing force  is  withdrawn.  Hard  steel  is  usually  quite  hard  to  mag- 
netize, but  it  retains  its  magnetism  quite  strongly.  Generally 
speaking,  the  harder  the  steel,  the  more  difficult  it  is  to  magnetize, 
and  the  more  strongly  it  retains  its  magnetism.  We  are  forced,  then, 
to  the  belief  that  there  is  some  force  that  prevents  the  molecular 
magnets  from  being  turned  away  from  any  position  which  they  hap- 
pen to  be  in.  This  hindering  force  is  called  coercive  force.  The 
coercive  force  of  soft  iron  is  quite  small  and  of  hard  steel  very  great. 
The  effect  of  the  coercive  force  is  counteracted  to  some  extent  by 
anything  which  is  likely  to  make  the  molecules  vibrate,  such  as 
rough  handling,  heating,  etc.  A  magnet  which  is  dropped  on  the 
floor  a  few  times  is  likely  to  lose  much  of  its  magnetism.  Heating 
to  a  red  heat  will  completely  demagnetise  a  magnet. 

If  a  magnet  is  magnetized  as  strongly  as  possible  it  is  said  to  be 
saturated.  When  a  magnet  is  saturated  it  will  generally  grow  weaker 
for  a  certain  time  after  magnetization,  till  it  finally  becomes  constant 
in  strength.  The  magnet  may  be  artificially  aged,  as  it  is  called, 
and  thus  brought  to  a  fairly  constant  strength,  by  immersing  i*  in 
steam  for  a  considerable  time. 

There  are  certain  similarities  which  may  readily  be  seen  between 
the  action  of  magnets  and  of  charged  bodies,  but  there  are  also 
many  marked  differences,  so  that  a  close  relationship  is  not  evident. 
There  is  a  remarkably  close  relationship  between  magnetism  and 
current  electricity,  which  will  be  taken  up  in  the  next  lesson. 

Before  leaving  this  lesson  it  is  necessary  to  examine  the  question 
of  what  causes  a  magnetic  needle  to  set  itself  north  and  south.  The 
mutual  action  of  magnets  which  has  been  explained,  leads  at  once  to 
the  conclusion  that  the  earth  is  a  great  magnet.  The  reason  for  the 
earth's  magnetism  is  not  known,  but  its  magnetic  strength  and  the 


31 


location  of  its  poles  have  been  carefully  determined.  One  magnetic 
pole  is  near  the  true  north  pole  of  the  earth.  This  is  the  one 
towards  which  the  north  pole  of  a  magnetic  needle  points.  The  loca- 
tion of  the  pole  varies  slightly  from  time  to  time,  but  the  actual 
variation  of  the  magnetic  needle  from  the  true  north,  or  its  declina- 
tion, and  the  rate  at  which  the  declination  changes  at  any  part  of  the 
earth,  may  be  determined  and  marked  on  a  map.  Such  maps  may 
be  used  for  correcting  the  indications  of  a  compass. 

Any  space  in  which  there  is  magnetism  and  consequently 
magnetic  force  is  called  a  magnetic  field,  or  afield  of  magnetic  force. 

The  magnitude  or  intensity  of  the  magnetic  force  at  any  point  is 
called  the  strength  of  the  field  at  that  point. 

The  theoretical  method  of  measuring  the  strength  of  a  field  is  by 
determining  the  force  which  a  magnet  pole  experiences  when  placed 
in  the  field.  A  magnet  pole  which  exerts  a  push  equal  in  magnitude 
to  the  force  called  a  dyne  upon  an  exactly  equal  pole,  when  the  two 
are  one  centimeter  (metric  measure)  apart,  is  called  a  magnet  pole  of 
unit  strength,  or  a  unit  pole.  The  strength  of  a  magnetic  field  is  given 
by  the  number  of  dynes  of  force  which  it  exerts  upon  a  magnet  pole 
of  unit  strength. 

If  an  independent  north  pole  could  be  placed  in  front  of  the 
north  pole  of  a  magnet  it  would  be  repelled  by  the  latter  pole  and 
attracted  by  the  south  pole  of  the  magnet.  This  would  cause  the 
independent  pole  to  move  away  from  the  magnet's  north  pole  and 
towards  its  south  pole,  but  as  it  moved  it  would  continually  change 
its  relative  distance  from  the  two  poles,  and  the  relative  magnitude 


c 
c 


FIG.  26.  FIG.  28. 

of  the  force  exerted  upon  it  by  the  two  poles  would  vary.  The 
direction  of  the  motion  of  the  independent  pole  would  depend  upon 
the  relative  direction  and  magnitude  of  the  forces  which  the  two 
poles  of  the  magnet  exerted  on  it  at  every  point.  The  actual  path 
would  be  a  curved  line  very  much  like  the  line  AB  in  Fig. 
26,  An  independent  south  pole  would  move  in  an  opposite  direction, 
of  course,  but  over  a  similar  path. 

As  already  explained  (Lesson  5,  page  30)  it  is  impossible  to  have 
an  independent  magnet  pole,  but  for  this  experiment  the  companion 
pole  may  be  sufficiently  far  removed  to  satisfactorily  show  the  action. 
A  shallow  glass  dish  containing  a  little  water  may  be  placed  over  a 
magnet.  By  properly  sticking  a  magnetized  sewing  needle  in  a  cork 


32 


FIG.  27. 

it  may  be  floated  upon  the  water  in  a  vertical  position  with  one  of 
its  poles  close  to  the  bottom  of  the  dish.  Then  the  upper  pole  will 
be  so  much  farther  away  from  the  magnet  than  the  lower  one  that 
the  latter  will  be  affected  by  the  force  due  to  the  magnet  almost  like 
an  independent  pole.  If  the  lower  pole  of  the  needle  is  a  north  pole 
it  will  tend  to  move  through  the  water,  when  placed  in  front  of  the 
north  pole  of  the  magnet,  in  a  curved  line  away  from  the  north  pole 
and  towards  the  south  pole.  If  the  lower  pole  of  the  needle  is  a 
south  pole  it  will  tend  to  move  from  the  south  pole  towards  the  north 
pole.  This  is  exactly  as  already  explained  for  an  independent  mag- 
net pole.  The  experiment  here  outlined  and  which  may  be  so 
readily  tried,  is  more  striking  when  the  magnet  is  a  strong  electro- 
magnet such  as  will  be  explained  later,  because  the  force  acting  on 
the  floating  needle  to  make  it  move  is  then  greater. 

The  direction  of  the  force  at  different  points  of  the  magnetic 
field  which  is  around  a  magnet  may  be  shown  by  another  simple 
experiment.  A  sheet  of  paper  may  be  laid  over  the  magnet  and  iron 
filings  sifted  over  it.  Now  if  the  paper  be  slightly  tapped  the  filings 
will  arrange  themselves  in  curved  lines  like  those  shown  in  Fig.  27, 
all  of  which  converge  towards  the  two  poles.  If  the  figure  were 
sufficiently  large  it  would  be  approximately  shown  that  every  line 
which  starts  out  from  one  pole  finds  its  way  round  to  the  other  pole. 
The  lines  of  iron  filings  may  be  easily  fixed  in  position  if  the  paper  is 
paraffined  before  using  it,  by  simply  passing  the  flame  of  a  Bunsen  gas 
burner  over  it.  This  softens  the  paraffine  and  the  bits  of  iron  stick 
fast. 

The  magnetic  field  exists  all  around  a  magnet  exactly  as  shown 
by  these  experiments  in  one  plane.  This  may  be  shown  by  hanging 
a  short  magnetized  sewing  needle  on  a  light  thread  and  bringing  it 
near  a  magnet.  It  will  take  a  position  at  every  point  so  that  its  direc- 
tion is  tangent  to  the  direction  which  a  line  of  iron  filings  would 
take  at  the  same  point.  The  reason  for  the  needle  taking  this  posi- 
tion is  because  its  north  pole  tends  to  go  one  way  and  its  south  pole 


33 


the  other,  so  that  the  needle  turns  around  until  the  pull  on  the  two 
poles  is  in  a  direct  line  through  the  length  of  the  needle.  The  iron 
filings  used  in  the  experiment  described  above  are  nothing  more  than 
little  magnets  caused  by  induction,  and  they  take  up  their  position 
for  the  same  reason  that  the  needle  does. 

It  must  be  remembered  that  in  all  cases  of  attraction  or  repulsion 
between  two  bodies  the  force  exerted  is  mutual,  and  either  body  will 
be  moved  if  not  too  firmly  fixed.  This  is  true  whatever  be 
the  cause  of  the  force,  as  for  instance,  electrification,  magnetism, 
gravity,  muscular  force,  or  any  other  cause.  The  fact  that  the 
.  action  is  mutual  may  be  proved  by  placing  a  bit  of  iron  on  a  cork 
floating  in  water  and  presenting  a  small  magnet  to  it.  The  iron  will 
be  attracted  by  the  magnet  and  the  cork  will  be  moved  through  the 
water  by  the  force  of  the  attraction.  Now  if  the  magnet  be  placed 
upon  the  cork  and  the  iron  be  brought  near  it,  the  attraction  between 
the  magnet  and  the  iron  will  again  move  the  cork.  Finally,  if  the 
iron  and  the  magnet  be  placed  on  separate  corks,  the  corks  will  move 
towards  each  other.  This  shows  that  \.\\z  force  is  mutual,  and  it  is 
also  possible  to  show  that  the  pull  is  always  equal  on  the  iron  and 
the  magnet. 

A  convenient  way  of  looking  upon  a  magnetic  field  is  to  con- 
sider it  a  space  which  is  more  or  less  filled  with  magnetic  lines  of 
force.  The  strength  of  field  may  be  represented  by  the  number  of 
lines  of  force  to  the  square  centimeter  (metric  measure).  Then  if 
the  strength  of  field  be  such,  for  instance,  that  a  unit  pole  when 
placed  in  it  experiences  a  force  of  ten  dynes,  we  may  consider  the 
field  as  having  ten  lines  of  force  per  square  centimeter.  These  lines 
of  force  no  more  actually  exist  than  do  definite  stream  lines,  or  lines 
of  flow,  exist  ;^i  water  which  is  flowing  around  in  a  tub,  but  the  idea 
based  on  this  assumed  existence  is  a  very  useful  and  practical  one. 

It  is  useful  to  define  the  positive  direction,  or  down  stream 
direction  as  it  were,  along  lines  of  force.  As  a  matter  of  convenience 
the  direction  along  the  line  of  force  outside  of  a  magnet  from  the 
north  pole  to  the  south  pole,  or  the  direction  in  which  an  independent 
north  pole  would  tend  to  move,  is  called  the  positive  direction.  It  is 
also  useful  to  consider  the  lines  of  force  as  continuing  through  the 
material  of  a  magnet  from  the  south  pole  to  the  north  pole,  so  that 
they  make  complete  curves,  as  shown  in  Fig.  28. 

From  what  we  have  learned  of  the  mutual  action  of  magnets  we 
can  now  see  that  when  a  magnet  is  placed  in  a  magnetic  field  it 
apparently  tends  to  set  itself  in  such  a  direction  that  its  own  lines  of 
force  where  they  are  within  its  body  are  parallel  with  the  lines  of 
force  of  the  external  field.  The  eifect  is  exactly  as  though  lines  of 
forceMend  to  turn  themselves  so  as  to  be  parallel  with  each  other  and 
in  the  same  direction. 

Copyright  1894, 


The  National  School  of  Electricity. 

REVIEW  OF  LESSON  V. 

Points  for  Review.     1.     What  is  the  magnetism  called  which    remains  in  a  piece 
of  steel  after  it  has  been  subjected  to  the  influence  of  a  magnet? 

2.  What  is  the  magnetism  called  which  appears  in  a  piece  of  iron  when  it  is  placed 
under  the  influence  of  a  magnet,  but  which  disappears  when  the  influence  is  withdrawn? 

3.  What  is  the  law  of  attraction  and  repulsion  of  magnet  poles? 

4.  What  are  the  two  poles  of  a  magnet  called,  and  why? 

5.  Why  does  a  magnet  always  attract  a  piece  of  iron? 

6.  Upon  what  does  the  magnitude  of  the  force  exerted  between  two  magnets  depend? 

7.  What    is    the    force   called   which   causes   the   magnetism  of  steel    to  become 
permanent? 

8.  How  may  a  magnet  be  "aged"? 

9.  Why  does  the  earth  cause  a  magnetic  needle  to  set  itself  north  and  south? 

10.  Is  the  magnetism  of  the  earth  constant  in  direction? 

11.  What  is  a  magnetic  field  ? 

12.  What  is  the  strength  of  a  magnetic  field  ? 

13.  What  will  a  magnet  do  if  it  is  placed  in  a  magnetic  field  ? 

14.  If  a  piece  of  iron  is  brought  near  a  magnet,  and  is  therefore  attracted  by  the 
magnet,  how  is  the  magnet  affected  ? 

15.  What  is  the  positive,  or  down  stream,  direction  along  the  lines  of  force  ? 

16.  How  do  lines  of  force  act  toward  one  another  ? 


LESSON   VI. 

THE  MAGNETIC  EFFECTS  OF   ELECTRIC  CURRENTS, 
AND  MAGNETIC  CIRCUITS. 

The  real  connection  which  exists  between  magnetism  and  currents 
of  electricity  was  not  made  generally  known  until  Oersted,  a  Danish 
scientist,  published  the  fact  in  1819  that  a  magnetic  needle  is  dis- 
turbed by  the  presence  of  an  electric  current  in  its  neighborhood. 
This  fact  had  really  been  discovered  earlier,  but  it  did  not  become 
generally  known.  It  had  also  been  known  that  under  some  conditions 
lightning  discharges  had  magnetized  steel  needles,  but  the  conditions 
had  not  been  successfully  reproduced  by  experimenters.  The  publi- 
cation of  a  series  of  experiments  by  Oersted  therefore  led  a  number 
of  eminent  scientists  to  turn  their  attention  during  the  early  part  of 
this  century  to  a  determination,  as  complete  as  possible,  of  the  exact 
relation  existing  between  electricity  and  magnetism. 

If  a  magnetic  needle  be  placed  above  or  below  a  wire  which 
carries  an  electric  current,  the  needle  will  turn  on  its  pivot  so  as  to 
set  itself  as  nearly  as  possible  at  right  angles  to  the  wire.  This  may 
be  readily  tried  by  connecting  a  short  piece  of  copper  wire  to  one  or 
two  cells  of  gravity  battery  and  holding  the  wire  above  the  needle 

35 


FIG.  29. 

(Fig.  29)  while  the  current  flows  through  it.  The  effect  on  the 
needle  may  be  made  most  evident  by  making  and  breaking  the 
electric  circuit,  which  will  cause  the  needle  to  swing  back  and  forth. 
The  current  in  the  wire  has  the  greatest  effect  in  causing  the  needle 
to  deflect  from  the  north  and  south  position  when  the  wire  also  lies 
in  a  north  and  south  direction — that  is,  when  the  wire  is  parallel 
with  the  needle. 

When  not  disturbed  by  other  magnetic  effects  the  needle  stands 
north  and  south  on  account  of  the  force  due  to  the  earth's  magnetism. 
When  the  electric  current  is  placed  so  as  to  flow  near  the  magnetic 
needle,  the  needle  is  affected  by  the  force  of  a  magnetic  field  ivhich 
is  set  up  by  the  current,  which  tends  to  make  the  needle  set  itself  at 
right  angles  to  the  wire  carrying  the  current.  The  needle  takes  an 
intermediate  position  where  the  effect  of  the  two  magnetic  forces 
balance.  Its  position  therefore  depends  upon  the  magnitude  of  the 
force  due  to  the  earth's  magnetism  and  the  direction  and  magnitude 
of  the  force  due  to  the  magnetism  set  up  by  the  current. 

Magnetism  set  up  by  an  electric  current  is  called  electromagnet- 
ism.  The  direction  of  the  magnetic  force  due  to  electromagnetism 
is  always  at  right  angles  to  the  direction  of  the  current  which  pro- 
duces the  magnetism,  and  the  lines  offeree  in  the  magnetic  field  due 
to  the  current  must  therefore  be  circles  surrounding  the  wire  which 
carries  the  current.  The  reason  why  a  magnetic  field  is  set  up  by 
an  electric  current  is  entirely  unknown;  merely  the  experimental 
fact  and  its  applications  are  known, 

The  strength  of  the  magnetic  field  at  any  point  due  to  an  electric 
current  near  by,  depends  directly  upon  the  strength  of  the  current 
and  upon  the  average  distance  of  the  current  from  the  point 

The  magnetic  field  which  surrounds  a  wire  when  a  current 
flows  in  it  may  be  shown  in  a  way  similar  to  that  used  to  show 
the  field  around  a  magnet.  A  stout  copper  wire  may  be  passed 
vertically  through  a  hole  in  a  horizontal  sheet  of  stiff  paper.  If  iron 
filings  be  sprinkled  upon  the  paper  they  will  arrange  themselves  in 
circles  around  tlje  wire  when  a  current  is  passed  through  it.  If  a 
small  magnetic  needle  or  compass  be  placed  on  the  paper  with  its 
center  over  a  line  of  filings,  the  needle  will  tend  to  stand  at  a  tan- 


FIG.  30. 

to  the  line  (Fig.  30).  An  independent  pole  would  tend  to  move 
continuously  around  the  wire  along  one  of  the  lines. 

The  direction  in  which  the  magnetic  needle  points  in  this  case 
depends  upon  which  side  of  the  wire  it  stands,  and  upon  the  direction 
in  which  the  current  flows  in  the  wire.  In  Fig.  30  it  is  evident 
from  the  position  of  the  magnetic  needles,  the  black  ends  of  which 
represent  north  poles,  that  the  positive  direction  along  the  lines  of 
force  is  there  left-handed  or  against  the  direction  in  which  the  hands 
of  a  clock  move.  If  the  direction  of  the  current  were  reversed,  the 
magnetic  needles  would  also  reverse  their  direction,  showing  that  the 
positive  direction  of  the  lines  of  force  has  a  fixed  relation  to  the 
direction  of  the  current. 

There  are  various  ways  of  remembering  this  relation.  One  is 
to  consider  an  ordinary  right-handed  screw  which  is  being  screwed 
into  or  out  of  a  nut  (Fig.  31).  If  an  electric  current  be  considered 
as  flowing  through  the  screw  in  the  direction  which  the  screw  moves 
through  the  nut,  then  the  positive  direction  of  the  lines  of  force 
is  shown  by  the  direction  in  which  the  screw  turns. 

Another  way  of  remembering  this  relation  is  according  to  a  rule 
proposed  by  Ampere,  after  whom  the  unit  of  electric  current  was 
named.  Suppose  a  man  lying  in  the  wire  with  his  head  down  the 
electric  stream  (swimming  with  the  electric  current);  then  if  he  faces 
a  magnetic  needle  placed  near  the  wire,  the  north  pole  of  the  needle 
will  tend  to  turn  towards  his  left  hand. 

This  relation  between  the  direction  of  the  current  flow  and  the 
deflection  of  a  magnetic  needle  gives  a  ready  method  for  determin- 
ing the  direction  of  the  current  in  a  wire,  the  only  instrument  which 
is  required  being  a  small  compass.  The  compass  may  be  placed 


37 


FIG.  32.  FIG.  34. 

under  the  wire  and  the  direction  towards  which  its  north  pole  turns 
noted.  Then  an  application  of  " Ampere's  rule"  gives  the  direction 
of  the  current. 

Since  we  have  seen  that  a  force  acting  between  two  bodies 
always  affects  them  both,  we  may  expect  that  a  wire  which  carries  a 
current  will  tend  to  move  when  brought  near  a  fixed  magnet.  This 
may  be  readilv  shown  by  suspending  a  very  flexible  conducting  wire 
near  a  fixed  magnet  (Fig.  32).  When  a  current  is  passed  through 
the  wire  it  will  wind  itself  around  the  magnet.  If  the  current  be 
reversed  the  wire  will  unwind  and  then  wind  around  the  magnet 
again,  but  in  the  opposite  direction. 

The  motions  of  the  wire  and  the  magnet  are  due  to  the  appar- 
ent tendency  of  magnetic  lines  of  force  to  move  out  of  a  position 
where  they  are  not  parallel and  into  a  position  where  they  are  parallel 
and  in  the  same  direction  (Lesson  5,  page  34) 

By  applying  Ampere's  rule  we  see  that  if  a  wire  carrying  a 
current  be  passed  above  a  magnetic  needle  and  then  turned  back  and 
passed  below  the  needle,  both  the  top  and  bottom  branches  tend  to 


38 


deflect  the  needle  in  the  same  direction,  so  that  the  effect  of  the  cur- 
rent on  the  needle  is  increased.  By  coiling  the  .wire  about  the  posi- 
tion of  the  needle  each  additional  turn  will  cause  an  additional  force 
to  deflect  the  needle.  In  this  way  the  magnetic  effect  of  a  current 
may  be  greatly  multiplied.  It  has  already  been  said  that  the  mag- 
netic force  at  a  point  due  to  a  current  near  it  depends  upon  the 
strength  of  the  current  (Lesson  6,  page  36).  We  now  see  that  when 
a  current  is  coiled  around  a  point  the  force  depends  upon  the 
strength  of  the  current  multiplied  by  the  number  of  turns  in  the  coil. 
The  product  of  the  current  by  the  turns  is  usually  called  current- 
turns  or  ampere-turns. 

When  a  wire  carrying  a  current  is  coiled  into  a  ring  or  helix, 
the  lines  of  force  which  surround  each  turn  seem  to  join  together  so 
that  they  belong  to  the  coil  or  winding  as  a  whole  (Fig.  33).  Such 
coils  are  often  called  solenoids.  Such  coils,  when  a  current  is  passed 
through  them,  exhibit  all  the  magnetic  effects  which  are  shown  by 
steel  magnets.  They  attract  and  repel  magnets  and  other  solenoids, 
and  attract  pieces  of  iron.  If  suspended  so  that  they  are  free  to 
swing,  they  turn  into  a  north  and  south  position  exactly  like 
magnets. 

This  magnetic  effect  of  coils  or  solenoids,  led  Ampere  to  suppose 
that  all  magnetism  is  caused  by  electric  currents.  He  therefore  sug- 
gested that  the  molecules  of  magnetic  materials,  and  possibly  of  all 
materials,  have  little  electric  currents  flowing  around  them  which 
make  them  into  magnets.  This  is  called  "Ampere's  theory"  of 
magnetism.  If  it  is  correct  it  gives  a  ready  explanation  of  why 
magnetism  is  found  in  various  materials,  but  it  still  leaves  unex- 
plained the  reason  for  the  electric  current  causing  magnetism. 
Ampere's  theory,  and  other  theories  of  magnetism  advanced  by  var- 
ious other  scientists,  have  been  before  the  scientific  world  for  many 
years,  but  their  correctness  has  not  yet  been  either  proved  or  dis- 
proved. We  are  therefore  forced  to  content  ourselves  with  the 
knowledge  that  the  molecules  of  magnetic  materials,  at  least,  are 
magnetic  (Lesson  5,  page  31). 

If  a  bar  of  hard  steel  be  placed  in  a  solenoid  through  which  a 
current  is  passing,  it  becomes  strongly  magnetized,  and  remains  per- 
manently magnetized  when  the  current  is  stopped  or  the  steel  is  with- 
drawn from  the  solenoid.  This  effect  is  exactly  the  same  as  would 
be  obtained  by  touching  the  steel  with  a  permanent  magnet,  but  the 
magnetic  effect  of  a  solenoid  with  many  turns  of  wire  may  be  made 
much  greater  than  any  permanent  magnet  and  the  steel  may  there- 
fore be  more  readily  saturated  by  the  solenoid. 

If  a  bar  of  soft  iron  be  placed  in  the  solenoid  it  becomes  even 
more  strongly  magnetized  than  the  steel,  when  the  current  is  turned 
on.  When  the  current  is  turned  off,  the  iron  loses  nearly  all  of*  its 
magnetism.  If  the  bar  is  very  soft  Swedish  iron  its  coercive  force  is 


39 


so  small  that  the  least  tap  shakes  practically  all  the  magnetism  out 
of  it.  Harder  and  less  pure  iron  retains  a  little  of  the  magnetism, 
the  amount  depending  upon  the  quality  of  the  iron.  The  magnetism 
which  is  retained  by  iron  after  it  has  been  magnetized  is  called  resid- 
ual magnetism. 

The  property  of  soft  iron  by  which  it  becomes  strongly  magnet- 
ized when  placed  within  a  solenoid  carrying  an  electric  current,  and 
then  loses  its  magnetism  upon  breaking  the  current,  was  discovered 
by  William  Sturgeon  in  1825.  An  arrangement  consisting  of  a  soft 
iron  core  which  is  surrounded  by  a  solenoid  or  winding  is  called  an 
electromagnet.  Electromagnets  are  of  the  greatest  value  in  the 
electrical  industries  because  they  can  be  built  of  practically  any  desired 
size  and  form,  and  of  enormous  strength.  The  magnets  of  com- 
mercial dynamos  and  electric  motors  are  always  electromagnets. 
Figure  34  shows  two  forms  of  horseshoe  electromagnets. 

At  the  time  of  the  discovery  of  the  electromagnet  nothing  was 
known  of  its  great  commercial  future;  but  it  was  welcomed  with  the 
highest  scientific  interest.  At  that  day  the  laws  of  electric  circuits 
were  unknown,  the  common  insulated  wire  of  today  was  not  made, 
and  the  manufacture  of  an  electromagnet  was  a  matter  of  much 
labor.  Moreover,  the  only  sources  of  current  were,  at  first,  plain 
zinc-copper  cells,  and  later,  Grove,  Daniell,  or  similar  types  of  galvanic 
cells.  Many  electromagnets  were  soon  made,  however,  and  their 
effects  were  carefully  studied  by  enthusiastic  scientists,  in  spite  of  the 
difficulties  to  be  overcome.  By  the  year  1845,  °nly  5°  years  ago,  the 
investigators  had  succeeded  in  overcoming  their  lack  of  experimental 
facilities  and  had  mapped  out  the  laws  of  magnetic  circuits  very  much 
as  we  know  them  at  the  present  time.  Thus  was  laid  the  foundation 
of  the  profession  of  electrical  engineering. 

From  what  has  preceded  we  may  see  that  a  solenoid  in  which  a 
current  flows  and  which  contains  a  soft  iron  core  is  a  stronger  magnet 
than  a  similar  solenoid  containing  a  hard  steel  core,  and  it  is  a  very 
much  stronger  magnet  than  a  similar  solenoid  containing  no  core. 
Remembering  that  according  to  our  ideas  of  lines  offeree,  the  strength 
of  the  magnetism  of  the  solenoid  and  core  depends  on  the  number  of 
lines  of  force  which  pass  through  the  solenoid ;  then  since  so  many 
more  lines  of  force  pass  through  a  solenoid  when  a  steel  bar  is  placed 
in  it  than  passed  through  it  when  the  space  within  the  solenoid  was 
simply  occupied  by  air,  we  may  conclude  that  lines  of  force  are  more 
readily  set  up  in  steel  than  in  air.  Since  a  soft  iron  core  causes  more 
lines  of  force  to  pass  through  a  solenoid  than  does  a  steel  core,  we 
may  also  conclude  that  lines  of  force  are  even  more  readily  set  up  in 
soft  iron. 

The  relative  ease  with  which  magnetic  lines  of  force  may  be  set 
up  in  a  body  is  called  its  permeability.  As  a  matter  of  convenience 
it  is  usual  to  say  that  the  permeability  of  air  is  unity  (i).  As  com- 


4,0 


pared  with  this,  the  permeability  of  soft  iron  may  be  enormously  great. 
In  some  cases  it  becomes  many  thousand  times  as  great  as  that  of  air. 

The  permeability  of  all  materials,  except  a  few  highly  magnetic 
ones,  is  very  nearly  unity.  The  proper  division  between  materials 
that  are  called  paramagnetic  and  those  that  are  called  diamagnetic 
(Lesson  5,  page  30)  depends  upon  whether  their  permeability  is  greater 
than  unity  or  is  slightly  less. 

We  may  divide  materials  into  good  conductors  of  magnetic  lines 
of  force  or  good  magnetic  conductors,  and  poor  magnetic  conductors. 
There  are  no  materials  which  we  may  look  upon  as  being  really 
magnetic  insulators,  as  we  may  look  upon  some  materials  as  being 
practical  insulators  of  electricity.  The  permeability  of  a  material 
may  be  called  its  specific  magnetic  conductivity,  or  the  magnetic  con- 
ducting power  of  a  block  of  the  material  which  is  one  centimeter  long 
and  has  an  area  of  one  square  centimeter.  The  actual  magnetic 
conducting  power  or  conductivity  of  a  piece  of  material  decreases 
with  the  length  and  increases  with  the,  cross-section  of  the  piece. 
This  may  be  likened  to  electrical  conducting  power  or  conductivity 
(Lesson  2,  page  13).  The  opposite  or  reciprocal  of  magnetic  conduct- 
ing power  may  be  called  magnetic  -resistance  or  reluctance,  and  any 
path  through  which  lines  of  force  pass  may  be  called  a  magnetic  cir- 
cuit. These  terms  will  be  seen  to  be  entirely  similar  to  the  terms 
applied  in  the  case  of  electric  currents. 

By  similarity  with  electric  circuits  we  may  say  that  it  takes  some 
force  to  set  up  lines  of  force  in  a  magnetic  field  or  magnetic  circuit. 
We  may  call  this  magnetomotive  force  or  magnetic  pressure,  terms 
which  are  similar  to  electromotive  force  and  electrical  pressure. 
The  number  of  lines  of  force  in  any  magnetic  circuit  is  equal  to 
the  magnetic  pressure  divided  by  the  reluctance  of  the  circuit.  It 
can  be  shown  mathematically  that  the  magnetic  pressure  in  a  com- 
plete magnetic  circuit  is  equal  to  the  number  of  ampere-turns  mul- 
tiplied by  a  constant  which  is  practically  equal  to  i%.  In  order  that 
the  .strongest  possible  magnetism  shall  be  produced  in  any  magnetic 
circuit  it  is  necessary  to  have  the  circuit  made  up  as  far  as  possible  of 
material  having  the  highest  permeability — that  is,  soft  iron — and  to 
arrange  as  many  ampere-turns  as  possible  to  set  up  the  magnetism. 

The  apparent  similarities  of  electric  and  magnetic  circuits,  and 
their  really  fundamental  differences,  will  be  taken  up  in  some  of  the 
later  lessons. 

Copyrighted,  1894, 


The  National  School  of  Electricity. 

REVIEW   OF   LESSON  VI. 

Points  for  Review      \.     How  is  a  magnetic  needle  affected  when  brought  near  an 
electric  current?     Why? 

2.  What  is  the  magnetism  called  which  is  set  up  by  an  electric  current? 

3.  How  may  the  relation  between  the  direction  of  lines  of  force  which  surround 
a  current,  and  the  direction  in  which  the  current  itself  flows,  be  remembered? 

4.  How   may  the  direction  in   which  a  current  flows  be  determined  through  the 
indications  of  a  compass  needle? 

5.  How  is  a  current  affected  by  the  presence  of  a  magnet? 

6.  How  does  coiling  a  wire  around  a  magnetic  needle  affect  the  magnetic  force 
which  the  current  in  the  wire  exerts  on  the  needle? 

7.  What  dees  the  phrase  "ampere-turns"  mean? 

8.  In  what  respects  are  solenoids  carrying  a  current  similar  to  steel  magnets? 

9.  What  is  Ampere's  theory  of  magnetism? 

10.  What  affect  does  a  solenoid  carrying  a  current  have  on  a  bar  of  steel  which  is 
placed  inside  of  it?     What  effect  does  it  have  on  a  bar  of  iron? 

11.  What  is  residual  magnetism? 

12.  What  is  an  electromagnet? 

13.  What  is  meant  by  the  magnetic  permeability  of  a  material?     What  is  its  prac- 
tical value  for  most  materials  taken  to  be?     How  does  its  value  for  most  materials  com- 
pare with  its  value  for  a  few  magnetic  materials? 

14.  What  is  meant  by  the  magnetic  reluctance  of  a  magnetic  circuit?  What  is  meant 
by  magnetomotive  force  or  magnetic  pressure? 

15.  What  arrangements  must  be  made  in  order  that  the  strongest  possible  magnet- 
ism may  be  set  up  in  a  magnetic  circuit? 


LESSON   VII. 

OHM'S  LAW  OF  THE  FLOW  OF  ELECTRICITY. 

When  water  is  forced  through  a  pipe  under  pressure  from  a 
pump  or  other  source  of  pressure,  the  stream  of  water  which  flows  is 
proportional  to  the  pressure  divided  by  the  frictional  resistance  which 
the  pipe  presents  to  the  flow  of  the  water.  In  the  same  way,  when 
a  current  of  electricity  flows  through  a  wire  under  the  pressure  from 
a  battery  or  other  source  of  electricity,  the  current  which  flows 
in  the  circuit  is  equal  to  the  pressure  divided  by  the  resistance  of  the 
circuit.  This  relation  between  electric  current,  pressure,  and  resis- 
tance is  called  Ohm's  Law,  after  the  name  of  the  German  scientist 
who  first  formally  announced  it.  The  relation  representing  Ohm's 
Law  is  often  written  C  =  |,  where  C,  E  and  R  stand  for  current, 
pressure  and  resistance.  This  is  a  very  good  form  in  which  to  com- 
mit the  relation  to  memory.  The  expression  as  written  may  be  read 
C  equals  E  divided  by  R.  From  the  relation  as  written  above  it  is 
evident,  also,  that  E  equals  C  times  R,  and  R  equals  E  divided  by 

42 


C.  Consequently  if  any  two  out  of  the  three  fundamental  electrical 
quantities  which  exist  in  a  circuit  are  given,  the  third  can  at  once 
be  calculated.  Thus,  if  a  16  candle-power  incandescent  lamp  is 
known  to  take  y2  an  ampere  when  connected  to  a  circuit  which  fur- 
nishes current  at  a  pressure  of  no  volts,  the  resistance  of  the  lamp 
when  in  operation  may  be  calculated  at  once  to  be  no  divided  by  ^, 
which  gives  the  resistance  as  220  ohms. 

In  this  example  we  have  assumed  that  the  source  of  electricity 
has  sufficient  capacity  to  keep  up  the  full  pressure  at  the  lamp  termi- 
nals when  current  is  flowing  through  the  lamp.  Sometimes  this  is 
not  the  case  on  account  of  the  resistance  to  be  found  in  the  source 
itself,  or  the  internal  resistance  of  the  source.  A  similar  condition 
is  frequently  met  when  a  pump  is  attached  to  a  large  hose.  When 
the  hose  nozzle  is  closed  the  pump  will  give  a  large  pressure,  but 
when  the  nozzle  is  opened  the  pressure  falls  because  the  pump  does 
not  have  sufficient  capacity  to  keep  up  the  supply. 

When  it  is  desired  to  determine  the  current  that  will  flow 
through  a  circuit  due  to  a  pressure  from  a  source  of  current  that  has 
an  appreciable  internal  resistance,  it  is  necessary  to  add  up  the  re- 
sistances of  all  parts  of  the  circuit  before  making  the  calculation. 
For  instance,  if  two  cells  of  battery  each  giving  a  pressure  of  i.i 
volts,  and  each  having  an  internal  resistance  of  3  ohms,  be  con- 
nected to  an  external  circuit  of  2.8  ohms  resistance,  then  the  total 
resistance  in  the  circuit  is  8.8  ohms  and  the  pressure  which  acts  to 
pass  current  through  the  circuit  is  2. 2  volts.  The  current  flowing 
under  these  circumstances  is  ^  ampere  (C— ER  or  ^—2.2/8.8). 

The  resistance  to  the  flow  of  water  through  a  pipe  is  a  surface 
or  "skin"  effect,  and  depends  upon  the  velocity  with  which  the 
water  flows,  the  number  and  form  of  the  bends  in  the  pipe,  the  form 
of  its  cross  section,  and  its  length.  The  true  electrical  resistance  of 
a  conductor  is  quite  different  from  this,  since  it  simply  depends  upon 
the  nature  of  the  metal  from  which  the  conductor  is  made,  the  area 
of  its  cross  section,  its  length,  and  its  temperature. 

The  greater  the  cross  section  of  a  conductor  the  greater  is  its 
electrical  conducting  power,  and  therefore  the  less  its  resistance; 
and  the  longer  the  wire  the  less  is  its  conducting  power,  and  there- 
fore the  greater  is  its  resistance.  The  cross  sections  of  the  ordinary 
cylindrical  wires  are  proportional  to  the  squares  of  their  diameters, 
and  consequently  the  conducting  powers  of  similar  wires  are  directly 
proportional  to  the  squares  of  their  diameters.  This  makes  the 
resistances  of  similar  wires  to  be  inversely  as  the  squares  of  their 
diameters.  For  instance,  if  a  certain  copper  wire  has  a  resistance  of 
one  ohm,  the  resistance  of  a  copper  wire  of  the  same  length  but  of 
twice  the  diameter  is  only  one-fourth  of  an  ohm,  since  the  square  of 
two  is  four. 

The  adopted  definition  of  the  value  of  the  ohm  is  based  upon 
this  property  of  electrical  resistance  depending  simply  upon  the 


43 


nature  of  the  metal  composing  the  conductor,  its  temperature,  its 
length  and  the  inverse  of  its  cross  section.  The  approved  definitions 
of  all  the  electrical  units  were  adopted  at  the  Electrical  Congress 
held  in  Chicago  in  August,  1893.  The  definition  of  the  unit  of 
resistance  makes  one  ohm  equal  to  the  resistance  of  a  column  of 
pure  mercury  which  is  106.3  centimeters  long  and  has  a  uniform 
cross  section  which  contains  14.4521  grammes  of  mercury,  the 
temperature  being  that  of  melting  ice.  This  gives  the  column  the 
uniform  cross  section  of  one  square  millimeter  (metric  system).  The 
ohm  as  thus  defined  is  called  the  International  Ohm  to  distinguish  it 
from  units  based  on  definitions  adopted  at  previous  electrical  con- 
gresses, and  which  differ  slightly  from  the  International  Ohm  and 
from  one  another,  exactly  as  different  kinds  of  quart  measures  differ 
from  one  another,  as  is  told  in  books  on  arithmetic.  It  is  generally 
believed  that  the  definitions  given  by  the  Chicago  Electrical  Congress 
will  be  universally  accepted  and  will  never  be  changed.  The  units 
by  which  electricity  is  measured  will  then  be  the  same  in  all  coun- 
tries. This  is  true  of  no  other  units  which  are  used  in  common 
measurements. 

Before  the  Chicago  Electrical  Congress  was  held,  the  funda- 
mental definition  of  the  ampere  had  usually  been  based  upon  the 
electromagnetic  effect  of  currents,  but  at  that  Congress  a  definition 
was  adopted  which  is  based  on  the  electrochemical  effect  of  currents. 
The  International  Ampere  as  thus  defined  is  the  steady  current  which 
deposits  silver  at  the  rate  of  .001118  grammes  per  second  from  a 
solution  of  silver  nitrate  in  water,  the  solution  being  of  a  fixed 
strength  to  make  sure  of  the  action  being  regular. 

In  order  that  the  fixed  relation  represented  by  Ohm's  L,aw(C— |) 
shall  hold  with  these  definitions,  the  definition  of  the  International 
Volt  by  the  Chicago  Congress  is  the  pressure  which  causes  a  current 
of  one  ampere  to  flow  through  a  resistance  of  one  ohm. 

Since  a  column  of  mercury  is  an  inconvenient  device  to  handle, 
standard  resistances  made  of  mercury  are  not  used  in  ordinary  meas- 
urements' of  electrical  resistance,  but  coils  of  German  silver  wire,  or 
other  wires  of  high  resistance,  are  used.  These  coils  are  carefully 
adjusted  in  resistance  to  a  desirable  number  of  ohms  and  they  can 
then  be  used  in  the  measurement  of  the  resistance  of  any  conductor 
according  to  methods  which  will  be  explained  in  later  lessons.  Mer- 
cury resistances  are  used  only  in  well-equipped  scientific  laboratories 
to  determine  the^real  resistances  of  the  common  wire  resistance  coils. 

The  measurement  of  electrical  currents  is  also  more  frequently 
carried  out  in  practical  tests  by  means  of  instruments  depending 
upon  the  magnetic  effect  of  the  currents,  than  according  to  the  means 
indicated  in  the  definition  of  the  ampere.  Methods  of  measurement 
based  on  the  electrochemical  effect  of  currents  are  very  valuable  for 
determining  whether  the  indications  of  electromagnetic  instruments 
are  correct. 


FIG.  35. 

On  page  4  of  Lesson  i  is  given  a  table  which  shows  the  compara- 
tive order  of  the  conducting  powers  of  various  materials.  It  is  seen 
that  metals  stand  at  the  head  of  the  list,  and  their  conducting  power 
is  so  much  better  than  that  of  other  materials  that  we  ordinarily 
speak  of  them  alone  as  the  conductors  of  electricity.  Amongst  the 
pure  metals  themselves  there  is  considerable  difference  in  conducting 
power,  while  mixing  impurities  in  metals  or  mixing  metals  to- 
gether generally  decreases  their  conducting  power.  The  following 
table  gives  a  number  of  the  better  known  metals  and  common  alloys 
in  the  approximate  order  of  their  conducting  powers.  The  figures 
at  the  right  hand  of  the  names  of  the  metals  show  the  average  con- 
ducting power  of  pure  metals  and  of  alloys  of  fixed  composition,  in 
percentages  of  the  conducting  power  of  pure  silver.  Pure  silver  is 
the  best  conductor  known,  but  the  table  shows  that  it  is  very  closely 
approached  by  pure  copper. 
Silver .  .  .  100.  Aluminum . .  54.  Wrought  Iron . .  16.  Lead ....  8. 

Copper...  97.     Zinc. 28.     Nickel 12.     Mercury.  1.6 

Gold 75.     Platinum  ...  17.     Tin 12.     Cast  Iron  3. 

Platinum  Silver  made  of  2  parts  platinum  and  i  part  silver  6.4 

German  Silver  made  of  5^  parts  copper,  2    parts  zinc,  2j4 

parts  nickel 3.5 

German  Silver  made  of  6  parts  copper,    2^  parts  zinc,  i^ 

parts  nickel , 5. 

German  Silver  made  of  5  parts  copper,  3  ^  parts  zinc,  i  ^ 

parts  nickel 7.5 

The  quality  of  a  metal  and  the  way  in  which  it  has  been 
handled  in  the  course  of  manufacture  affects  the  conducting  power 
to  a  considerable  degree.  Pure  copper  that  comes  from  the  ore  of 
the  Lake  Superior  copper  mines  has  a  little  higher  conductivity  than 
that  coming  from  the  Arizona  mines.  Annealed  metals  (that  is, 
metals  which  have  been  softened  and  toughened  by  slow  cooling  from 
a  high  temperature)  generally  have  a  slightly  greater  conductivity 
than  hardened  metals,  and  wrought  metals  than  cast  metals. 


If  two  wires  be  connected  in  parallel  (that  is,  so  that  a  current 
divides  between  them  as  shown  in  Fig.  35)  the  current  flowing  in  each 
is  equal  to  the  pressure  between  their  common  terminals  divided  by 
their  individual  resistances.  For  instance,  if  the  two  wires  have 
resistances  of  4  and  6  ohms  respectively  and  the  pressure  between 
their  terminals  (the  points  A  and  B,  Fig.  35)  is  12  volts,  the  cur- 
rent flowing  through  the  first  wire  is  12/4=3  amperes  and  that  through 
the  second  is  12/6=2  amperes. 

We  have  already  seen  that  the  current  due  to  a  fixed  pressure 
which  flows  through  any  resistance  is  inversely  proportional  to  the 
resistance.  Accordingly  the  currents  flowing  through  the  two 
wires  of  the  example  should  be  in  the  proportion  of  j{  and  ^.  This 
is  true,  since  3  is  ^-  of  12  and  2  is  ^  of  12. 

The  total  current  flowing  through  the  circuit  containing  the  two 
wires  in  parallel  is  evidently  2  plus  3,  or  5  amperes.  Since  the  pres- 
sure causing  these  5  amperes  to  flow  through  the  wires  is  12  volts, 
the  resistance  of  the  circuit  between  A  and  B,  or  the  joint  resistance 
of  the  two  wires  in  parallel,  must  be  12/5  or  2.4  ohms.  This  may  be 
conveniently  calculated  directly  from  the  conductivities,  which,  it 
will  be  remembered,  are  reciprocal  or  inverse  to  the  resistances  (lyes- 
son  2,  page  13).  The  conductivity  of  the  first  wire  is  therefore  % 
and  that  of  the  second  wire  is  %.  The  joint  capacity  of  two  or  more 
pipes  which  deliver  water  between  two  tanks  is  equal  to  the  capaci- 
ties of  all  the  separate  pipes  added  together.  In  the  same  way  the 
joint  conducting  power  of  electric  circuits  which  are  connected  in 
parallel,  or  divided  circuits,  as  they  are  often  called,  is  equal  to  the 
conducting  powers  of  the  parts  added  together.  The  joint-conduct- 
ing power  or  conductivity  in  the  example  is  therefore  *^  P^us  Yt>  or 
512.  The  resistance  of  the  divided  circuit  is  the  inverse  of  this, 
which  is  equal  to  12/5  or  2.4,  as  previously  calculated. 

This  shows  that  simply  adding  together  the  resistances  of  the 
individual  parts  of  a  circuit  will  not  always  give  the  total  resistance 
of  the  circuit.  In  fact,  such  an  addition  gives  the  total  resistance 
only  when  all  the  individual  resistances  belong  to  parts  of  the  circuit 
which  are  connected  in  series  (Lesson  7,  page  43).  When  part  of  the 
total  circuit  is  made  up  of  conductors  in  parallel  it  is  necessary  to 
first  calculate  the  joint  resistance  of  that  part  and  then  add  that  to 
the  resistance  of  the  remainder  of  the  circuit.  It  is  easily  seen  that 
the  joint  resistance  of  conductors  in  parallel  is  equal  to  the  resistance 
of  a  single  conductor  with  which  they  might  be  replaced  without  chang- 
ing the  total  resistance  of  the  circuit. 

The  total  resistance  of  a  circuit  made  up  of  parts  connected  in 
series  is  equal  to  the  sum  of  the  individual  resistances  of  all  ihe  parts. 
The  total  resistance  of  a  circuit  made  up  of  parts  connected  in  par- 
allel is  equal  to  the  reciprocal  of  the  total  conductivity  of  the  circuit, 
and  the  total  conductivity  is  equal  to  the  sum  of  the  individual  conduc- 
tivities of  the  parts. 


FIG.  36.    . 

Circuits  are  sometimes  spoken  of  as  simple  circuits  when  the 
parts  are  all  in  series,  and  compound  or  derived  circuits,  when  the 
parts  are  in  parallel.  Parallel  connection  is  sometimes  called  connec- 
tion in  multiple  or  multiple  arc. 

In  Fig.  36  is  shown  a  circuit  which  contains  a  part  composed  of 
two  conductors  in  parallel.  Suppose  that  the  resistances  in  ohms  of 
the  different  parts  are  as  marked,  then  the  total  resistance  of  the  cir- 
cuit is  12  ohms.  If  the  pressure  developed  by  each  of  the  twc  cells, 
which  are  represented  by  the  usual  sign  =,  is  1.2  volts,  the  current 
flowing  through  the  circuit  is  2.4/12  =  .2  amperes. 

A  little  consideration  of  what  precedes  will  show  that  when  two 
wires  of  equal  resistance  are  connected  in  parallel,  their  joint  resist- 
ance is  just  half  as  great  as  the  resistance  of  either  wire.  If  three 
wires  of  equal  resistance  are  connected  in  parallel,  their  joint  resist- 
ance is  one-third  as  great  as  the  resistance  of  one  of  the  conductors, 
and  so  on.  If  the  wires  of  equal  resistance  were  connected  in  series 
instead  of  in  parallel,  the  total  resistances  would  be  two,  three,  and 
so  on,  times  as  great  as  a  single  wire. 

A  simple  rule  for  calculating  the  joint  resistance  of  two  wires 
which  are  connected  in  parallel  is  to  multiply  together  the  indi- 
vidual resistances  of  the  wires  and  divide  this  product  by  the  sum  of 
the  individual  resistances.  This  conies  directly  from  the  laws  of  the 
electric  current  and  the  resistances  of  divided  circuits,  but  it  is  gen- 
erally simpler,  as  already  said,  to  consider  the  conductivities  when 
calculating  joint  resistances  of  parallel  circuits. 

When  a  wire  is  connected  in  parallel  with  another  it  is  often 
called  a  shunt,  because  it  switches  off  or  shimts  a  part  of  the  current 
from  the  other  wire.  The  wire  to  which  a  shunt  is  attached  is  said 
to  be  shunted.  Special  shunts  put  up  in  boxes  are  frequently  used 
to  protect  electrical  instruments  which  are  required  for  electrical 


47 


measurements,  by  shunting  a  known  part  of  the  current  around  the 
instruments  when  they  might  be  injured  if  the  total  current  passed 
through  them. 

Since  Ohm's  Law  shows  that  the  electrical  pressure  between 
two  points  in  a  circuit  is  equal  to  the  current  flowing  in  the  circuit 
multiplied  by  the  resistance  of  the  part  of  the  circuit  between  the 
points  (Lesson  7,  page  42),  we  may  say  that  the  pressure  along  a  wire 
falls  in  proportion  to  the  resistance  passed  over.  Thus,  suppose  the 
terminals  of  a  copper  wire  of  uniform  cross  section  and  10  feet  long, 
be  connected  to  the  poles  of  an  electric  battery  furnishing  a  pressure 
of  two  volts.  Now  since  equal  lengths  of  the  uniform  wire  may  be 
considered  as  having  equal  resistances  and  all  parts  of  the  wire  carry 
the  same  current,  the  electrical  pressure  measured  between  the  mid- 
dle of  the  wire  and  one  end  must  be  equal  to  the  pressure  measured 
between  the  middle  and  the  other  end,  and  this  must  also  be  equal 
to  one  volt  or  one-half  the  total  pressure  measured  between  the  ends 
of  the  wire.  In  the  same  way  the  pressure  measured  across  any  por- 
tion of  the  wire  bears  the  same  proportion  to  the  two  volts'  total  pres- 
sure, as  the  length  of  the  portion  bears  to  the  whole  length  of  the 
wire. 

If  one  end  of  the  wire  while  still  connected  to  the  battery  is 
connected  to  earth  (by  connecting  to  a  water  or  gas  pipe)  it  may  be 
considered  as  being  at  zero  pressure;  then  the  other  end  is  at  an 
actual  pressure  of  two  volts.  (The  difference  of  pressure  between 
the  two  ends  of  the  wire  was  considered  before  without  taking  into 
account  their  actual  pressures.  The  same  thing  is  often  done  in  con- 
sidering the  flow  of  water  or  gas  through  a  pipe.)  The  middle  of 
the  wire  is  at  a  pressure  of  one  volt,  while  2^/2  feet  or  one-quarter  the 
length  of  the  wire  from  the  upper  end  the  pressure  is  i  ^  volts,  and 
7  ^  feet  or  three-quarters  of  the  length  of  the  wire  from  the  upper 
end  the  pressure  is  y2  volt. 

If  the  wire  were  not  of  a  uniform  cross  section,  or  were  com- 
posed in  different  parts  of  different  metals,  then  the  resistance  of 
equal  lengths  would  no  longer  be  the  same.  The  pressures  meas- 
ured across  the  portions  of  the  wire  would  no  longer  be  proportional 
to  the  length  of  the  portions,  but  would  be  proportional  to  their 
resistances,  as  before. 

The  general  rule  may  therefore  be  written  as  a  result  of  Ohm's  Law; 
the  electrical  pressure  along  a  conductor  through  which  a  given  current 
flows,  falls  directly  as  the  resistance  passed  over.  The  same  rule  holds 
in  the  case  of  gas  or  water  flowing  through  a  pipe.  Suppose  it  requires 
ten  pounds  pressure  to  cause  500  gallons  of  water  to  flow  per  minute 
through  a  certain  straight  pipe  200  feet  long.  If  the  pipe  be  cut  in 
half,  5  pounds  pressure  is  sufficient  to  pass  the  same  amount  of  water 
through  either  half.  If  pressure  gauges  are  attached  with  proper 
precautions  to  the  pipe  at  intervals  of  20  feet,  each  gauge  will  show 


48 


a  pressure  of  one  pound  less  than  the  preceding  one,  when  taken  in 
the  direction  of  the  current.  This  shows  that  the  pressure  falls 
directly  as  the  resistance  passed  over,  as  in  the  case  of  the  electric 
current. 

Reference  has  already  been  made  to  the  effect  of  temperature  on 
the  resistance  of  metals.  THe  resistance  of  most  metals  increases  as 
the  temperature  rises,  but  in  the  case  of  a  few  metals,  the  most  im- 
portant of  which  are  some  alloys  and  carbon,  the  resistance  falls  as 
the  temperature  increases.  The  fall  of  the  resistance  of  carbon  as 
the  temperature  rises,  is  sufficiently  great  to  reduce  the  working 
resistance,  or  hot  resistance,  of  an  incandescent  lamp  filament  to  only 
about  one-half  the  resistance  which  it  has  when  at  the  usual  atmos- 
pheric temperature.  The  resistance  of  liquids  and  of  most  insulating 
materials,  as  far  as  they  are  measurable,  decreases  as  the  temperature 
rises. 

The  resistance  of  most  pure  metals  seems  to  change  at  nearly 
the  same  rate,  namely:  about  .4  of  i  per  cent  per  degree  of  the  centi- 
grade thermometer  scale  or  .22  of  i  per  cent  per  degree  of  the 
Fahrenheit  thermometer  scale.  (One  centigrade  degree  is  equal  to  | 
of  a  degree  of  the  Fahrenheit  scale.)  This  is  a  fairly  accurate  value 
of  the  temperature  coefficient  of  ordinary  copper.  A  change  of  .4  of 
one  per  cent  per  centigrade  degree  means  a  change  of  one  per  cent 
in  resistance  up  or  down  for  every  2^  degrees  centigrade  when  the 
temperature  varies  up  or  down.  This  is  also  nearly  equivalent  to 
one  per  cent  for  every  five  degrees  of  the  Fahrenheit  or  common  ther- 
mometer scale. 

The  temperature  coefficient  of  alloys  depends  very  much 
upon  the  composition  of  the  mixture.  In  general,  German 
silver  may  be  taken  to  have  a  temperature  coefficient  about  one- 
tenth  as  great  as  that  of  copper.  The  temperature  coefficients  of 
the  alloys,  whose  comparative  conductivities  are  given  in  the  first 
part  of  this  lesson,  are  compared  below  with  that  of  copper: 

CENT.  FAHR. 

Copper .40  .22 

Platinum  Silver .030  .017 

German  Silver .033  .018 

German  Silver .036  .020 

German  Silver .040  .022 

The  two  columns  of  figures  in  this  table  show  the  approximate 
temperature  coefficient  of  the  metals  expressed  as  the  percentage 
change  of  resistance  per  degree  centigrade  and  Fahrenheit. 

Copyrighted,  1894, 


The  National  School  of  Electricity. 

REVIEW  OF  LESSON  VII. 

Points  for  Review.     1.     What  is  Ohm's  law? 

2.  Suppose  a  pressure  of  ten  volts  is  maintained  between  the  terminals  of  a  wire 
the  resistance  of  which  is  four  ohms,  how  much  current  flows? 

3.  If  a  current  of  five  amperes  flows  through  a  wire  which  has  a  resistance  of  12 
ohms,  what  is  the  pressure  between  the  terminals  of  the  wire? 

4.  If  a  battery  of  five  gravity  cells,  each  of  which  gives  a  pressure  of  1.08  volts  and 
has  an  internal  resistance  of  four  ohms,  be  connected  in  series  with  an  external  resist- 
ance of  seven  ohms,  what  current  flows  through  the  circuit? 

5.  If  two  cells  which  respectively  give  pressures  of  1.8  volts  and  1.08  volts  are  con- 
nected to  a  circuit  in  opposition  (that  is,  with  their  poles  connected  so  that  they  tend  to 
send  currents  in  opposite  directions),  and  a  current  of  .4  amperes  flows,  how  much  cur- 
rent will  flow  if  the  cells  are  connected  to  the  same  circuit  properly  in  series? 

6.  If  two  copper  wires  of  equal  length  have  resistances  of   four  and  nine   ohms 
respectively,  and  the  diameter  of  the  first  is  one-eighth  inch,  what  is  the  diameter  of 
the  other? 

7.  If  the  resistance  of  -a  coil  of  wire  is  found  to  be  105  ohms,  and  a  piece  of  the 
same  wire  which  is  ten  feet  long  has  a  resistance  of  1.5  ohms,  how  many  feet  of  wire  are 
contained  in  the  coil? 

8.  Why  is  the  volt  defined  as  the  pressure  which  is  required  to  pass  a  current  of 
one  ampere  through  a  resistance  of  one  ohm? 

9.  Why  are  mercury  resistances  not  used  in  every  day  measurement  of  resistances? 

10.  How  does  the  conductivity  of  copper  compare  with  that  of  other  metals? 

11.  Why  is  wire  made  from  Lake  Superior  copper  preferred  for  electrical  purposes 
by  users  of  copper  wire? 

12.  Suppose  an  electric  battery  is  connected  to  two  external  circuits  in  parallel,  one 
of  them  having  a  resistance  of  20  ohms  and  the  other  a  resistance  of  40  ohms,  what  pro- 
portion of  the  total  current  will  flow  through  each  circuit? 

13.  Suppose  the  battery  of  the  third  example  gives  a  pressure  of  20  volts  and  has  an 
internal  resistance  of  6%  ohms,  how  much  current  flows  through  the  battery,  and  how 
much  through  each  of  the  external  circuits? 

14.  What  is  the  total  resistance  of  a  series  circuit  made  up  of  parts  having  the  fol- 
lowing resistances:  1st  part,  4  ohms;  2nd  part,  2  ohms;  3rd  part,  2/4  ohms? 

15.  What  is  the  resistance  of  a  circuit  made  up  of  the  same  parts  in  parallel? 

16.  Suppose  three  wires,  each  having  a  resistance  of  4  ohms,  are  first  connected  in 
series  and  then  connected  in  parallel,  what  is  their  joint  resistance  in  each  case? 

17.  If  a  uniform  wire  20  feet  long  measures  1  ohm,  what  is  the  fall  of  pressure  per 
foot  when  one  ampere  flows  through  it?     What  is  the  fall  of  pressure  per  foot  when  two 
amperes  flow  through  it,  the  resistance  being  assumed  to  remain  constant? 

18.  V/hat  effect  does  temperature  have  on  the  resistance  of  most  metals?     What 
effect  does  it  have  on  the  resistance  of  carbon,  liquids,  and  most  insulators? 

19.  What  is  the  approximate  temperature  co-efficient  of  copper? 

20.  How  many  Fahrenheit  degrees  change  of  temperature  is  required  to  change  the 
resistance  of  a  copper  wire  one  per  cent? 

50 


LESSON  VIII. 

HEATING  EFFECTS  OF  ELECTRIC  CURRENTS. 
MISCELLANEOUS  EFFECTS  OF  ELECTRIC  CURRENTS. 

When  one  coulomb  of  electricity  is  passed  through  a  wire  under 
the  pressure  of  one  volt,  a  certain  amount  of  work  is  done,  exactly 
as  a  certain  amount  of  work  is  done  when  a  gallon  of  water  is 
pumped  through  a  pipe  under  a  pressure  of  one  pound.  In  the  case  of 
the  water  the  work  done  is  measured  \n foot-pounds^  which  means  that 
a  force  equivalent  to  one  pound  has  been  moved  through  a  distance 
of  one  foot.  In  order  to  determine  the  foot-pounds  of  work  done  in 
pumping  water,  the  pressure  under  which  the  water  is  pumped  must 
be  converted  into  its  equivalent  feet  of  head  and  the  quantity  of 
water  must  be  given  in  pounds.  A  pressure  of  one  pound  is  equiva- 
lent to  the  head  of  a  column  of  water  2  ^  feet  high,  and  the  weight 
of  a  gallon  of  water  is  about  8^  pounds.  Consequently  if  one  gallon 
of  water  be  passed  through  a  pipe  under  a  pressure  of  one  pound, 
about  19^2  foot-pounds  of  work  is  done.  (2^3x8^  = about  19^2.) 
In  the  same  way  when  one  coulomb  of  electricity  is  passed 
through  a  wire  under  a  pressure  of  one  volt,  the  amount  of  work 
done  is  called  one  joule,  after  the  name  of  Joule,  a  great  English 
scientist  and  engineer. 

As  a  general  thing  we  do  not  care  to  pump  a  single  gallon  of 
water  through  a  pipe,  but  we  wish  to  pump  a  given  number  of  gal- 
lons per  minute.  In  this  case  for  each  gallon  passed  per  minute 
through  the  pipe  under  a  pressure  of  one  pound,  about  19^  foot- 
pounds of  work  must  be  done  every  minute.  Suppose  it  is  desired 
to  pump  120  gallons  (1,000  pounds)  of  water  per  minute  through  a 
pipe  under  a  head  of  33  feet,  the  work  required  to  do  this  is  33,000 
foot-pounds  per  minute.  The  rate  at  which  work  is  done,  that  is,  the 
amount  done  in  a  given  time,  is  called  poiver.  In  dealing  with 
mechanical  power  it  is  divided  into  units  called  horse-power.  A 
horse-power  is  equal  to  33,000  foot-pounds  of  ivork  done  per  minute,  so 
that  in  the  last  example  exactly  one  horse-power  is  required  to  move 
the  water. 

The  horse-power  of  a  water  fall  is  calculated  in  a  way  which  is 
similar  to  the  preceding  examples.  Suppose  a  stream  discharges 
480  gallons  or  4,000  pounds  of  water  per  minute  over  a  fall  25  feet 
high,  the  power  of  the  water  is  100,000  foot-pounds  per  minute  or  a 
little  over  three  horse-power.  The  horse-power  of  a  steam  engine  is 
also  calculated  in  a  similar  manner.  For  instance,  in  an  engine 
which  is  supplied  with  steam  which  exerts  an  average  pressure  on 
the  piston  of  40  pounds  per  square  inch  along  the  whole  stroke  and 
the  piston  of  which  has  a  surface  of  TOO  square  inches,  the  total  pres- 
sure exerted  by  the  steam  on  the  piston  is  4,000  pounds.  If  the 

51 


stroke  of  the  engine  is  one  foot,  the  piston  moves  two  feet  per  revolu- 
tion, and  consequently  the  steam  exerts  8,000  foot-pounds  of  work 
in  each  revolution.  If  the  engine  runs  at  250  revolutions  per  minute 
the  work  done  by  the  steam  is  2,000,000  foot-pounds  per  minute  or  just 
a  little  more  than  60  horse-power.  This  is  called  the  indicated  horse- 
power of  the  engine.  Most  of  it  is  available  for  driving  machinery, 
but  a  portion  is  used  in  overcoming  the  friction  ot  the  engine  itself. 

We  have  seen  that  when  a  coulomb  of  electricity  is  sent 
through  a  wire  under  a  pressure  af  one  volt,  an  amount  of  work  is 
done  which  is  called  a  joule;  also  that  a  current  of  one  ampere  is  a 
current  which  conveys  one  coulomb  per  second.  (Lesson  2,  page 
12.)  Consequently  when  a  current  of  one  ampere  is  passed  through 
a  wire  under  a  pressure  of  one  volt,  the  amount  of  work  done  is  equal 
to  one  joule  per  second.  This  represents  a  certain  amount  of  power 
which  is  called  a  watt,  after  James  Watt,  a  great  English  engineer 
and  the  inventor  of  the  modern  steam  engine.  The  power  repre- 
sented by  one  watt  is  equal  to  one  seven  hundred  and  forty-sixth 
part  of  a  horse-power,  or  there  are  74.6  watts  in  a  horse-power.  In 
speaking  of  the  power  of  electrical  machinery,  it  has  become  usual 
to  use  the  electrical  term  ivatt,  and  for  a  larger  and  frequently  more 
convenient  unit  the  kilowatt  is  used.  This  is  equal  to  1,000  watts, 
or  about  i  %  horse-power. 

When  an  electric  current  flows  through  a  circuit,  the  power 
used  in  the  circuit  is  equal  to  the  current  multiplied  by  the  total 
pressure  causing  the  current  to  flow.  That  is,  the  power  in  watts  is 
equal  to  the  current  in  amperes  multiplied  by  the  pressure  in  volts, 
or  P=C  E.  Part  of  this  power  may  be  used  in  causing  electro- 
chemical action,  by  charging  a  storage  battery  for  instance,  or  it 
may  be  used  in  driving  machinery  through  the  medium  of  an 
electric  motor,  but  some  of  the  power  is  always  used  in  overcoming 
the  resistance  of  the  wires  which  convey  the  current.  This  is  some- 
what similar  to  the  use  of  some  of  the  indicated  power  of  an  engine 
in  overcoming  the  friction  of  the  engine  itself. 

When  mechanical  power  is  used  in  overcoming  friction  or  other 
forms  of  resistance,  it  is  not  lost  but  is  converted  into  an  equivalent 
amount  of  heat.  A  general  law  may  be  stated  that  energy  (that  is, 
the  capability  of  doing  work)  is  never  destroyed,  but  it  may  be  trans- 
formed from  one  form  to  another.  This  is  called  the  Law  of  the  Con- 
servation of  Energy.  When  power  is  transformed  from  one  form  to 
another,  there  is  always  some  loss  of  the  amount  of  useful 
power.  The  apparently  lost  power  has  not  been  destroyed,  but  has 
been  converted  into  heat.  For  instance,  when  the  mechanical  power 
conveyed  by  a  running  belt  is  changed  by  means  of  a  dynamo  of  sat- 
isfactory size  into  electrical  power,  about  ten  per  cent  of  the  availa- 
ble power  is  lost.  That  is,  the  electrical  power  delivered  by  the 
dynamo  is  about  ten  per  cent  less  than  the  mechanical  power  which 


is  given  to  the  dynamo.  This  difference  has  not  been  destroyed, 
but  has  been  converted  into  heat  in  overcoming  the  friction  of  the 
dynamo  bearings,  the  resistance  of  the  wire  windings  of  the  dynamo, 
and  in  other  ways.  A  dynamo  which  is  in  operation  is  always  found 
to  be  warmer  than  the  surrounding  air,  which  shows  that  some  of 
the  power  delivered  to  it  is  changed  into  heat  that  goes  to  warm  the 
machine.  The  usefulness  of  this  amount  of  power  is  therefore  lost, 
but  the  energy  is  not  destroyed. 

The  power  which  is  used  in  overcoming  the  electrical  resistance 
of  a  wire  when  a  current  is  passed  through  it  is  converted  into  heat 
which  warms  the  wire.  The  heat  produced  is  proportional  to  the 
number  of  watts  required  to  overcome  the  resistance  of  the  wire,  and 
this  is  equal  to  the  difference  of  pressure  at  the  terminals  of  the  wire 
multiplied  by  the  current  flowing  in  it  (P  =  C  E),  provided  all 
the  power  expended  in  that  part  of  the  circuit  is  used  in  heating  the 
wire.  According  to  Ohm's  Law,  pressure  is  equal  to  current  times 
resistance,  or  E  =  C  R.  Consequently  C  times  E  is  equal  to  C 
times  OR,  or  C  squared  times  R.  Hence  the  power  required  to  over- 
come the  resistance  of  a  wire  is  equal  to  the  square  of  the  current 
multiplied  by  the  resistance,  or  P  =  C  E  =  C  2R.  By  again  substi- 
tuting according  to  Ohm's  Law,  it  may  also  be  shown  that  the 
power  lost  in  a  wire  is  equal  to  the  pressure  squared  divided  by  the 
resistance,  or  P  =  C  E  =  C  2R  =  E  2/R. 

Since  the  portion  of  the  available  electrical  power  of  a  circuit 
which  is  lost  in  heating  the  conductors  is  equal  to  the  current 
squared  times  the  resistance  of  the  conductors,  it  is  often  spoken  of 
as  the  C  squared  R  loss. 

According  to  the  Law  of  the  Conservation  of  Energy,  which 
was  experimentally  proved  by  Joule,  for  every  unit  of  work  trans- 
formed into  heat  there  is  an  equivalent  amount  of  heat  produced. 
Consequently  if  we  have  two  wires,  the  first  of  which  has  double  the 
resistance  of  the  second,  and  equal  currents  are  passed  through  them, 
the  power  lost  and  the  heat  produced  in  the  first  will  be  twice  as 
great  as  in  the  second. 

It  is  possible  to  measure  an  electric  current  by  the  heat  pro- 
duced when  it  is  passed  through  a  known  resistance.  This  is 
usually  done  in  an  instrument  called  a  calorimeter  (Fig.  37),  which 
is  a  vessel  containing  water  or  some  other  liquid  in  which  the 
resistance  is  immersed.  The  vessel  is  usually  double  walled  or 
arranged  in  some  other  way  so  that  it  will  not  lose  heat  rapidly  by 
radiation  into  the  air,  A  thermometer  is  immersed  in  the  liquid  to 
determine  its  rise  of  temperature  due  to  the  heat  given  it  from  the 
wire.  The  amount  of  heat  which  is  required  to  raise  the  tempera- 
ture of  a  gramme  of  water  one  degree  of  the  centigrade  scale  is  called 
a  calorie.  The  number  of  calories  given  to  the  water  in  the  calori- 
meter by  the  wire,  is  determined  from  the  amount  of  water  and  its 


53 


rise  in  temperature,  proper  corrections  being  made  for  the  effect  of 
the  vessel.  The  experiments  of  Joule  and  of  Rowland,  an  American 
scientist,  have  shown  that  the  work  represented  by  one  joule  is 
equivalent  to  the  heat  represented  by  practically  .24  of  a  calorie. 
Consequently,  the  total  number  of  calories  of  heat  produced  in  one 
second  by  the  current  passing  through  the  wire  in  the  calorimeter  is 
equal  to  .24  C  2R.  The  total  heat  produced  in  any  time  is  also  equal 
to  .24  C  2R  multiplied  by  the  number  of  seconds  in  the  time.  This 
may  be  written  in  the  form  H  =.24  C  2R  T.  By  determining  the 
total  heat  produced  in  the  calorimeter  in  a  fixed  time,  when  the 
current  is  passed  through  a  known  resistance,  the  value  of  the  cur- 
rent may  be  determined.  The  square  of  the  current  is  equal  to  the 
calories  divided  by  .24  times  the  resistance,  multiplied  by  the  time 
in  seconds. 


FIG.  37. 

It  may  be  seen  from  what  precedes  that  one  ampere  flowing 
through  a  resistance  of  one  ohm  expends  continuously  a  power  of  one 
watt  and  produces  .  24  calories  of  heat  every  second. 

The  expansion  or  lengthening  of  a  wire  when  it  is  heated  by  a 
current  passing  through  it  may  also  be  used  to  measure  the  current, 
as  will  be  fully  explained  later. 

The  actual  rise  of  temperature  on  the  part  of  a  wire  when  a  cur- 
rent passes  through  it,  depends  upon  several  things  in  addition  to  the 


amount  of  heat  produced  in  it.  A  long,  thick  wire  and  a  short,  thin 
wire  of  the  same  material,  and  having  the  same  resistance,  will  come 
to  very  different  temperatures  when  equal  currents  are  passed  through 
them.  If  there  is  sufficient  difference  in  their  diameters,  the  thin 
wire  may  become  red  hot  on  account  of  the  passage  of  a  current  which 
is  only  sufficient  to  make  the  thick  wire  appreciably  warm. 

When  a  current  passes  through  a  wire  a  certain  amount  of  heat 
is  produced  during  every  second  which  the  current  flows.  For  a  short 
time  after  the  current  is  started  the  wire  rises  in  temperature,  and 
finally  reaches  a  certain  fixed  temperature.  When  the  temperature 
becomes  fixed  it  is  evident  upon  a  little  thought  that  as  much  heat 
must  leave  the  wire  by  radiation  to  surrounding  objects,  convection 
by  air  currents,  or  conduction  to  objects  touching  the  wire,  as  is  pro- 
duced by  the  flow  of  the  current.  If  more  heat  is  given  to  the  wire 
than  is  carried  off  by  these  means,  its  temperature  must  rise,  and  if 
on  account  of  a  decrease  in  the  current  the  amount  of  heat  given  to 
the  wire  is  less  for  a  time  than  the  amount  given  off,  the  temperature 
must  fall  until  the  two  are  equal  again.  The  capability  of  a  wire  to 
get  rid  of  heat  by  radiation  and  convection  depends  upon  the  color 
and  condition  of  its  surface,  and  also  roughly  upon  the  extent  of  the 
surface.  The  amount  of  heat  which  leaves  any  surface  in  a  second 
also  depends  upon  the  number  of  degrees  by  which  its  temperature 
is  higher  than  that  of  the  air  and  surrounding  objects.  The  amount 
of  heat  which  is  required  to  bring  a  wire  to  a  given  temperature  also 
depends  upon  the  capacity  of  the  material  for  holding  heat,  or  its 
specific  heat  as  it  is  called.  Consequently  the  actual  temperature  to 
which  any  wire  will  rise  when  carrying  a  certain  current,  can  be 
exactly  determined  only  by  trying  the  experiment. 

The  fact  that  the  ability  of  a  wire  to  emit  heat  is  directly  depend- 
ent upon  the  extent  of 'its  surface,  causes  a  wire  with  an  insulating 
covering  to  remain  cooler  in  the  open  air  than  a  similar  wire  without 
the  covering,  but  carrying  an  equal  current.  This  seems  at  first 
sight  exactly  opposed  to  the  facts  as  seen  in  covered  boiler  pipes. 
There  is  no  contradiction,  however,  because  the  thickness  of  the 
insulation  is  entirely  comparable  with  the  diameter  of  the  wire,  and 
the  outside  surface  of  the  insulation  is  therefore  so  much  greater  than 
that  of  the  wire,  that  the  additional  surface  more  than  makes  up  for 
the  difficulty  which  the  heat  experiences  in  getting  through  the 
.insulation,  and  the  heat  finds  it  easier  to  leave  the  insulated  wire. 
This  effect  is  most  decidedly  shown  when  the  outer  surface  of  the 
insulator  is  black.  When  steam  pipes  are  covered  for  the  purpose 
of  retaining  their  heat,  the  thickness  of  the  covering  is  thin  com- 
pared with  the  diameter  of  the  pipes,  so  that  the  outside  surface  of 
the  covered  pipes  is  not  much  greater  than  the  surface  of  the  pipes 
when  bare.  Consequently  the  effect  of  the  thickness  of  the  cover- 
ing which  is  placed  in  the  path  of  the  heat  as  it  leaves  the  pipe  is 


55 


greater  than  the  effect  of  the  increased  surface,  and  the  heat  finds  it 
more  difficult  to  leave  a  covered  pipe.  When  wires  are  closed  up  in 
mouldings  or  under  plaster,  as  is  often  the  case  with  the  electric 
light  wires  in  buildings,  they  become  very  much  hotter  than  when 
exposed  in  such  a  way  that  they  may  be  cooled  by  air  currents. 


Electric  currents  cause  various  effects  besides  those  of  electro- 
chemistry, electromagnetism  and  electric  heating.  These  effects  are 
of  various  kinds,  but  of  small  commercial  importance,  and  in  most 
cases  seem  to  be  due  to  some  action  of  the  current  upon  the  mole- 
cules of  the  material  through  which  it  flows.  Some  of  the  effects  are 
undoubtedly  due  to  electrochemical  action,  though  they  have  often 
been  attributed  to  some  unknown  action  of  the  current.  The  only 
one  of  these  effects  which  is  of  sufficient  importance  outside  of  the 
field  of  purely  speculative  science  to  require  attention,  is  the  physio- 
logical action  of  the  current.  Galvani  accidentally  discovered  this 
action  through  some  experiments  performed  upon  frogs.  His  dis- 
covery has  been  followed  up  by  many  scientists  down  to  the  present 
day,  and  a  vast  array  of  facts  has  been  determined  relating  to  the 
effects  of  currents  on  living  organisms.  The  researches  of  these 
scientists  have  shown  that  protoplasm,  which  is  the  fundamental 
basis  of  all  living  bodies,  has  the  power  of  contracting  when  an  elec- 
tric current  passes  through  it.  Moreover,  a  living  animal  nerve  is 
always  excited  to  action  by  the  passage  through  it  of  an  electric 
current  from  an  external  source.  If  the  terminals  of  a  battery  cell 
be  touched  to  the  tongue,  a  peculiar  taste  may  be  noticed.  This  taste 
may  also  be  caused  by  laying  a  copper  and  a  silver  coin  upon  the 
tongue  with  their  edges  touching.  In  this  case  a  current  is  set  up 
through  the  metals,  the  saliva  of  the  mouth  serving  as  the  fluid.  If 
the  terminals  of  a  battery  cell  be  touched  to  the  temples,  or  so  that 
the  current  flows  from  the  forehead  to  the  hand,  flashes  of  light  are 
frequently  noticeable,  due  to  the  excitation  of  the  nerves  of  the  eye 
by  the  current.  In  the  same  way  the  nerves  of  smell  and  hearing 
may  be  excited. 

When  a  sufficiently  powerful  electric  current  is  passed  through 
the  ordinary  nerves,  a  feeling  of  tickling,  pricking,  or  pain  may  be 
observed.  If  the  current  is  sufficiently  strong,  it  may  cause  a  very 
painful  muscular  contraction,  and  if  excessive,  the  current  may  cause 
death.  The  muscular  effect  due  to  a  strong  current  is  ordinarily 
called  a  shock.  The  severity  of  shock  depends  upon  the  electrical 
pressure  which  causes  the  current  to  flow  through  the  body,  but  it 
also  depends  largely  upon  the  physiological  condition  of  the  person 
who  receives  the  shock. 


It  has  been  found  that  electric  currents  naturally  exist  in  the 
living  muscles  and  nerves  of  animals,  and  that  muscular  exertion 
seems  to  cause  them.  These  currents  disappear  with  the  death  of 
the  animal,  which  probably  shows  that  the  electric  currents  have 
some  function  in  the  action  of  the  nervous  system. 

The  physiological  action  of  electric  currents  gives  a  good  basis 
for  their  use  in  the  treatment  of  some  diseases,  and  they  have  been 
used  with  marked  success  in  some  cases.  This  question  will  be  fully 
treated  in  special  lessons,  and  it  is  sufficient  to  add  here  that  electric 
treatment  should  never  be  applied  except  under  the  immediate  direc- 
tion of  a  competently  trained  physician.  The  indiscriminate  use  of 
electrical  treatments  of  any  kind  is  likely  to  do  more  harm  than  good. 

Copyrighted,  1895, 


ITT 
•JRNIA- 


The  National  School  of  Electricity. 

.REVIEW  OF  LESSON  VIII. 

Points  for  Review.     1.     What  is  a  foot-pound?     What  is  a  joule? 

2.  What  is  a  horse-power?     What  is  a  watt?     What  is  a  kilowatt? 

3.  How  many  watts  are  in  a  horse-power? 

4.  What  is  the  reason  that  the  bearings  of  machinery  become  warm? 

5.  When  a  current  of  10  amperes  flows  through  a  resistance  of  2  ohms,  how  much 
power  is  used?     How  much  heat  is  produced  per  second? 

6.  If  a  current  of   10  amperes  is  caused    to  flow  through    a  circuit  under  a  pressure 
of  100  volts,  how  much  power  is  used?     If  part   of  the  power  is  used  in   electrochemical 
operations  but  the  total  resistance  of  the  circuit  is  5  ohms,  what  proportion  of  the  power 
is  used  in  the  C2  R  loss? 

7.  Why  does  a  black  covered  insulated  wire  remain  cooler  when  carrying  a    certain 
current  than  a  bare  wire  of  equal  size  carrying  the  same  current  and  exposed  to  the  same 
conditions? 

8.  What  is  the  effect  of  a  current  of  electricity  when  passed  through  the  nerves  of 
animals?     What  is  the  muscular  effect  ot  a  strong  current? 

9.  Is  it  safe  to  use  patent  electro-medical  devices,  or  receive  electrical  treatment 
from  untrained  hands? 


IX. 

GALVANOMETERS  AND  VOLTAMETERS. 

Instruments  for  detecting  and  measuring  electric  currents,  the 
indications  of  which  are  dependent  upon  the  deflection  of  a  magnetic 
needle  caused  by  the  magnetic  effect  of  the  current  flowing  in  a  coil 
which  surrounds  the  needle,  are  called  galvanometers.  These  instru- 
ments are  made  in  a  great  variety  of  forms  and  are  widely  used  for 
measurements  in  laboratories  and  shops. 

In  most  forms  of  galvanometers  the  magnetic  needle  is  placed  at 
the  center  of  a  coil  of  wire.  This  coil  may  have  a  great  number  of 
turns  of  fine  wire,  in  which  case  the  galvanometer  is  sensitive,  that 
is,  the  needle  is  appreciably  deflected  by  a  very  small  current;  or 
the  coil  may  have  but  few  turns  of  thick  wire,  in  which  case  the  gal- 
vanometer is  intended  for  use  with  comparatively  large  currents.  In 
many  cases  the  coil  of  the  galvanometer  is  placed  so  that  it  stands  in  an 
exact  north  and  south  position  (that  is,  in  the  magnetic  meridian)  like 


the  needle.  The  magnetic  force  due  to  the  coil,  which  is  at  right  angles 
to  its  wire  (Lesson  6,  page  36),  is  then  at  right  angles  to  the  magnetic 
force  of  the  earth  and  also  to  the  length  of  the  needle.  When  the 
coil  is  in  this  position  a  current  in  the  coil  exerts  its  greatest  force 
to  deflect  the  needle. 

When  a  galvanometer  is  connected  in  a  circuit  the  presence  of  a 
current  is  shown  by  the  deflection  of  the  needle.  The  direction  of 
the  current  is  shown  by  the  side  towards  which  the  north  pole  of  the 
needle  moves  (Lesson  6,  page  37).  The  strength  of  the  current  is 
indicated  by  the  amount  of  the  needle's  deflection,  since  the  position 
which  the  needle  takes  depends  upon  the  relative  magnitude  of  the 
magnetic  forces  due  to  the  current  and  the  earth  (Lesson  6,  page  35). 
The  earth's  magnetism  can  be  considered  to  be  approximately  con- 
stant at  any  fixed  point. 

When  the  diameter  of  the  galvanometer  coil  is  much  greater 
thau  the  length  of  the  needle,  the  tangents  of  the  angles  through 
which  the  needle  is  deflected  by  various  currents  are  proportional  to 
the  currents.  Such  a  galvanometer  is  called  a  tangent  galvanometer. 
(Fig.  38.)  Other  galvanometers  in  which  the  coil  is  moved  so  as  to 
bring  the  needle  back  to  zero  (Fig.  39),  are  called  sine  galvanometers, 
because  the  sine  of  the  angle  through  which  the  coil  is  moved  is  proper-  * 
tional  to  the  current  causing  the  deflection.  In  some  rough  galvan- 
ometers a  pointer  is  attached  to  the  needle,  and  the  deflection  is  read 
off  on  a  divided  circle  over  which  the  pointer  moves.  The  circle  is 
usually  divided  uniformly  in  degrees.  For  exact  measurements  such 
a  method  of  reading  deflections  is  not  sufficiently  accurate  and  re- 
flecting galvanometers  are  used.  (Fig.  40.)  A  small  mirror  is  attached 
to  the  magnet  in  these,  and  the  deflections  are  read  off  by  means  of  a 
small  telescope  which  shows  the  reflection  in  the  mirror  of  a  stationary 
scale.  When  the  needle,  with  its  mirror,  moves,  the  reflection  of 
the  scale  as  seen  in  the  telescope  appears  also  to  move  and  the  deflection 
of  the  needle  is  thus  determined.  Instead  of  using  a  telescope  and 
scale  as  is  usually  done  in  America,  a  lamp  and  scale  (Fig.  41)  may 
be  used,  as  is  usually  done  in  England.  In  this  case  abeam  of  light 
from  a  lamp  which  is  placed  behind  a  slit  in  front  of  the  gal- 
vanometer is  reflected  by  the  mirror  upon  a  scale,  where  it  shows  as 
a  spot  of  light.  When  the  needle  with  its  mirror  is  deflected,  the 
spot  of  light  moves  along  the,  scale,  thus  showing  the  magnitude  ot 
the  deflection.  This  is  a  very  convenient  arrangement  to  use  when 
testing  must  be  done  in  dark  rooms  or  vaults,  but  it  cannot  be  used 
in  a  light  place. 

The  support  of  the  needle  is  sometimes  in  the  form  of  a  finely 
wrought  pivot,  and  the  needle  is  sometimes  set  with  an  agate  or 
ruby  center  so  that  it  may  move  easily.  The  friction  of  the  finest 
pivot,  however,  is  so  great  that  it  destroys  the  sensitiveness  of  a  fine 


59 


FIG.  39.  FIG.  42.  FIG.  43. 

galvanometer,  so  that  in  all  fine  galvanometers  the  needles  are  sus- 
pended by  means  of  &  fibre  which  is  usually  made  of  unspun  cocoon 
silk.  The  fineness  of  this  fibre  depends  upon  the  weight  of  the 
needle  with  its  mirror,  and  it  is  sometimes  so  fine  that  it  can 
scarcely  be  seen.  The  length  of  the  suspension  varies  from  a  small 
fraction  of  an  inch  to  many  inches. 

It  is  often  convenient  to  make  the  needle  of  a  galvanometer 
independent  of  the  direction  of  the  earth's  magnetism,  or  to  vary  the 
strength  of  the  directive  force,  that  is,  the  force  which  holds  the 
needle  in  the  magnetic  meridian.  For  this  purpose  galvanometers 
are  generally  arranged  with  one  or  more  directive  magnets  or 
controlling  magnets.  One  is  shown  as  a  curved  bar  placed  on  a 
stem  above  the  galvanometers  of  Fig.  40.  By  varying  the  position 


60 


FIG.  41. 

of  the  magnet  with  respect  to  the  needle,  the  needle  may  be  controlled 
as  desired,  and  the  galvanometer  may  be  set  in  any  desired  position. 

In  order  that  a  galvanometer  may  be  made  very  sensitive,  it  is 
desirable  to  make  the  controlling  force  very  weak — in  some  cases 
much  weaker  than  that  due  to  the  earth's  magnetism.  Conse- 
quently the  effect  of  the  earth's  magnetism  must  be  overcome.  For 
this  purpose  what  are  known  as  astatic  needles  are  used.  These  con- 
sist of  a  pair  of  needles  of  practically  equal  size  and  magnetic  strength 
which  are  fastened  to  a  light  thin  wire,  one  above  the  other  so  that 
their  north  poles  point  in  exactly  opposite  directions.  It  is  usual  to 
arrange  a  coil  of  wire  for  each  needle,  so  that  the  galvanometers 
have  two  coils.  In  some  very  sensitive  galvanometers  there  are 
eight  needles  arranged  astatically,  and  eight  coils. 

The  forms  in  which  galvanometer  needles  are  made  are  quite 
various.  Some  needles  are  in  the  form  of  a  partially  split  bell,  one 
side  being  the  north  pole  and  the  other  being  the  south  pole.  Other 
needles  are  made  of  flat  discs  or  rings,  which  are  so  magnetized  that 
a  portion  of  the  edge  serves  as  the  north  pole  and  an  opposite  por- 
tion as  the  south  pole.  The  commonest  form  of  needle  is  one  made 
up  of  several  little  magnets,  made  from  a  watch  spring,  laid  side  by 
side  with  their  poles  all  the  same  way.  These  are  usually  fastened 
to  the  back  of  the  galvanometer  mirror  or  to  a  little  disc  of 
aluminum. 

There  is  a  very  convenient  form  of  galvanometer  in  which  the 
coil  is  suspended  so  as  to  move  in  the  magnetic  field  of  a  strong 
Ahorse  shoe  magnet.  (Fig-  42).  In  this  instrument  the  relations  of 


/c 


coil  and  magnet  are  practically  the  reverse  of  those  in  the  common 
galvanometers.  This  is  called  a  d'1  Arsonval  galvanometer,  after  a 
French  scientist  who  put  it  in  useful  form.  The  suspension  of  the 
coil  of  a  d'Arsonval  galvanometer  must  be  arranged  so  that  the 
current  may  get  into  and  out  of  the  coil.  The  coil  is  therefore  often 
supported  between  stretched  phosphor-bronze  wires  which  are  con- 
nected to  it  at  the  top  and  bottom  and  which  serve  as  leads  for  the 
current.  Sometimes  the  coil  is  suspended  on  a  silver  wire  by  means 
of  which  the  current  can  enter  the  coil,  and  a  wire  at  the  bottom  of 
the  coil  dips  into  a  bottle  of  mercury  so  that  the  current  can  get  out. 

One  reason  that  a  d'Arsonval  galvanometer  is  convenient  tor 
general  use  is  because  it  v&dttid beat,  that  is,  when  the  coil  is  deflected 
it  goes  at  once  to  its  position  without  a  tedious  period  of  swinging 
back  and  forth.  Ordinary  galvanometers  may  be  made  more  or  less 
dead  beat  by  surrounding  the  needle  with  a  ball  of  copper,  or  by 
attaching  to  the  suspensions  wings  of  mica  or  aluminum  which  are 
enclosed  in  a  small  chamber. 

In  order  that  a  galvanometer  may  be  used  to  actually  measure 
currents  in  amperes,  the  constant  of  the  galvanometer  must  be 
known,  or  the  galvanometer  must  be  calibrated  or  standardized. 

When  the  deflections  of  a  galvanometer  bear  some  fixed  relation 
to  the  currents  causing  the  deflections,  it  is  said  to  have  a  constant 
For  instance,  in  the  case  of  a  tangent  galvanometer  the  current 
.which  causes  a  certain  deflection  of  the  needle  is  given  in  amperes 
by  multiplying  the  tangent  of  the  angle  of  deflection  by  the  constant 
of  the  galvanometer.  The  constant  of  a  tangent  galvanometer  may 
be  directly  calculated  when  the  coil  is  circular,  and  its  diameter  and 
number  of  turns  and  the  strength  or  the  earth's  magnetism  are 
known.  The  constant  may  also  be  determined  by  passing  a  current 
of  known  strength  through  the  galvanometer  and  observing  the 
deflection. 

When  the  deflections  of  the  galvanometer  are  not  known  to  bear 
a  fixed  relation  to  the  currents  causing  them,  the  galvanometer 
must  be  experimentally  calibrated.  That  is,  currents  of  various 
known  strengths  must  be  passed  through  the  galvanometer  and  the 
deflections  observed.  These  observations  may  be  set  down  in  a  table 
so  as  to  be  used  in  future  work  with  the  galvanometer,  or  the  obser- 
vations may  be  plotted  in  a  curve  on  cross  ruled  paper.  Such  a 
calibration  curve  is  often  convenient  since  the  value  of  a  current 
corresponding  to  any  deflection  may  be  at  once  determined  from  it. 

When  galvanometers  are  used  simply  for  the  detection .  of 
currents  or  for  comparing  the  relative  magnitude  of  currents  as  is 
frequently  the  case,  a  calibration  is  unnecessary. 

An  instrument  for  measuring  currents  by  means  of  their  electro- 
chemical action,  which  is  often  used  in  calibrating  galvanometers, 
is  called  a  voltameter.  We  have  alreadv  seen  that  chemical  action 


goes  on  in  a  battery  cell  when  a  current  is  passed  in  either  direction 
through  the  cell,  and  that  the  amount  of  the  action  is  proportional 
to  the  number  of  coulombs  of  electricity  passed  through  the  cell 
(Lesson  4,  page  24).  The  chemical  action  in  a  voltameter  is 
similar  to  that  which  takes  place  in  a  voltaic  cell,  but  both  plates 
are  of  the  same  material  and  there  is  therefore  no  tendency  to  set  up 
a  current  due  to  the  direct  action  of  the  cell. 

An  electric  current  seems  to  flow  through  some  liquids  in  a 
different  way  from  that  in  which  it  flows  through  solid  conductors. 
In  fact,  liquids  may  be  divided  into  three  classes  on  the  ground  of 
their  action  when  subjected  to  the  effect  of  an  electric  pressure:  i. 
Those  that  appear  as  insulators  of  a  high  grade,  such  as  paraffine  oil, 
turpentine,  etc.  2.  Those  which  conduct  like  solids,  without  appar- 
ent chemical  action,  such  as  mercury,  metals  in  a  melted  condition, 
etc.  3.  Those  in  which  chemical  decomposition  occurs  when  a 
current  flows  through  them,  such  as  solutions  of  acids,  or  metallic 
compounds,  and  some  melted  solid  compounds. 

Liquids  of  the  latter  class  are  called  electrolytes,  and  the  process 
of  their  decomposition  by  electrochemical  action  is  called  electrolysis. 
A  cell  in  which  electrolysis  is  carried  on  is  generally  called  an 
electrolytic  cell,  or  when  the  electrochemical  action  is  used  to  de- 
termine the  strength  of  the  current  flowing  through  the  cell,  it  is 
called  a  voltameter  as  already  stated.  The  plates  of  an  electrolytic 
cell  are  called  electrodes.  The  positive  electrode  (the  one  at  which 
the  current  enters)  is  often  called  the  anode,  and  the  negative  elec- 
trode, the  cathode.  The  products  of  the  electrolysis  are  often 
called  ions. 

The  earliest  form  of  voltameter  is  one  in  which  sulphuric  acid 
greatly  diluted  by  water,  is  electrolyzed.  This  is  called  a  water 
voltameter.  A  form  of  water  voltameter  is  shown  in  Fig.  43.  When 
this  is  to  be  used,  diluted  acid  is  poured  into  the  funnel  at  the  back, 
and  rises  to  the  top  of  the  two  arms  in  front,  if  the  stop  cocks  at 
their  tops  are  open.  After  the  tubes  are  filled  the  cocks  are  closed, 
and  the  current  is  passed  between  the  platinum  electrodes,  EK. 
The  electrochemical  action  set  up  by  the  current  causes  oxygen  to  go 
to  the  positive  pole  or  anode,  and  hydrogen  to  go  to  the  negative  pole 
or  cathode.  The  gases  rise  in  the  tubes  above  their  respective  elec- 
trodes, and  displace  the  water.  While  the  direct  action  of  the  current 
is  to  cause  a  decomposition  of  sulphuric  acid,  additional  chemical  ac- 
tion occurs  which  makes  the  total  action  equivalent  to  the  decompo- 
sition of  water.  Water  is  composed  of  two  parts  by  bulk  of  hydrogen  to 
one  part  of  oxygen,  and  consequently  the  tube  over  the  cathode  collects 
twice  as  much  gas  as  that  over  the  anode.  If  a  steady  current  is  passed 
through  such  a  voltameter  for  a  given  number  of  seconds  the  strength 
of  the  current  can  be  determined  from  the  amount  of  the  gases  col- 
lected per  second.  For,  the  number  of  coulombs  of  electricity  passed 


63 


through  the  voltameter  is  determined  from  the  amount  of  the  gases 
collected  and  their  electrochemical  equivalent  (Lesson  4,  page 
24).  The  number  of  coulombs  passed  through  the  circuit  per 
second  is  equal  to  the  current  in  amperes. 

A  water  voltameter  is  not  a  very  convenient  or  satisfactory  in- 
strument, and  voltameters  in  which  the  electrolytes  are  solutions  of 
the  salts  of  metals  are  preferred  for  real  measurements.  When  such 
a  solution  is  electrolysed  between  plates  of  the  metal  contained  in  the 
solution,  the  solution  is  decomposed;  the  metal  from  the  solution  goes 
ivith  the  current  to  the  cathode  where  it  is  deposited  and  the  acid 
part  of  the  compound  goes  to  the  anode,  which  it  attacks  and  forms  a 
new  portion  of  the  compound.  The  cathode  should  therefore  be  ex- 
pected to  gain  exactly  as  much  metal  from  the  deposit  as  the  anode 
loses  by  the  attack  of  the  acid.  This  would  be  true  if  no  chemical 
action  occurred  except  that  directly  caused  by  the  current.  It  is  a 
fact  that  the  character  of  a  deposited  metal  often  varies  with  the  cur- 
rent strength  by  means  of  which  it  is  deposited,  or  the  strength  of 
the  solution  used  as  the  electrolyte.  Copper  is  sometimes  deposited 
in  the  form  of  a  black  powder  instead  of  a  smooth,  bright  layer  of 
metal.  Silver  is  often  deposited  in  crystals  which  build  across  the 
electrolyte  between  the  electrodes.  Tin  forms  a  "tree"  of  tin  crys- 
tals when  deposited  from  a  tin  chloride  solution,  the  branches  of 
which  spread  out  from  the  electrode  through  the  solution.  The 
greatest  care  must  be  used  to  get  satisfactory  measurements  by  means 
of  a  voltameter.  The  loss  of  the  anode  is  seldom  as  reliable  a  meas- 
ure of  the  current  as  the  gain  of  the  cathode,  because  bits  of  metal 
are  liable  to  be  loosened  up  on  the  former  and  fall  off,  and  the  anode 
also  often  suffers  from  oxidation. 

When  a  silver  voltameter  is  used  for  the  measurement  of  a  cur- 
rent, as  is  assumed  in  the  definition  of  the  ampere  (Lesson  7,  page  44), 
the  electrolyte  is  a  solution  of  the  nitrate  of  silver  of  fixed  strength. 
The  cathode  is  usually  a  platinum  bowl  upon  which  the  silver  is 
deposited,  and  the  anode  is  a  wire  or  plate  of  pure  silver  which  is 
wrapped  in  filter  paper  to  keep  bits  of  silver  from  dropping  onto  the 
cathode.  Before  a  measurement  of  current  is  to  be  made,  the  cathode 
is  very  accurately  weighed,  the  solution  is  then  poured  in  and  the 
anode  put  in  place.  The  current  is  turned  on  and  continued  for  a 
desirable  number  of  seconds.  It  is  then  stopped,  the  cathode  is  care- 
fully washed  and  dried,  and  finally  again  weighed  with  great  care. 
From  its  gain  in  weight  the  value  of  the  current  is  determined. 

On  account  of  the  expense  of  the  silver  consumed  and  the  care 
required  in  using  a  silver  voltameter,  it  is  not  satisfactory  for  meas- 
uring currents  exceeding  about  one  ampere.  For  larger  currents,  a 
voltameter  having  copper  plates  and  a  solution  of  copper  sulphate  for 
electrolyte  is  generally  used.  The  meter  used  by  Edison  companies 
to  determine  the  Quantity  of  electricity  delivered  per  month  to  cus- 


tomers,  usually  consists  of  a  voltameter  with  amalgamated  zinc  plates 
and  an  electrolyte  of  zinc  sulphate. 

The  weight  in  grammes  of  different  metals  deposited  by  one 
ampere  in  one  second  (that  is,  their  electrochemical  equivalent,  Les- 
son 4,  page  24,)  is  given  below. 

ELECTROCHEMICAL 
EQUIVALENT. 

Hydrogen oooo  1 04 

Silver , 001118 

Copper 000328 

Zinc °°°337 

Copyrighted,  1894, 


The  National  School  of  Electricity. 

REVIEW  OF   LESSON  IX. 

Points  for  review.     1.     What  is  a  galvanometer? 

2.  What  is  the  fundamental  difference  in  the  construction  of  galvanometers  for  use 
With  very  small  currents  and  with  large  currents? 

3.  How  is  the  presence  and  direction  of  a  current  in  a  circuit  shown  by  a  galvan- 
ometer? 

4.  How  is  the  strength  of  a  current  indicated  by  a  galvanometer? 

5.  What  is  a  tangent  galvanometer?     What  is  a  reflecting  galvanometer? 

6.  How  are  the  needles  of  fine  galvanometers  suspended? 

7.  What  is  a  d'Arsonval  galvanometer? 

8.  What  is  meant  by  the  term  "dead  beat?"  What  is  meant  by  the  term  "calibrate?" 

9.  What  is  a  voltameter? 

10.  What  is  meant  by  the  terms  electrolyte,  electrolysis,  electrode? 

11.  What  occurs  when  a  solution  of  a  metallic  salt  is  electrolyzed? 

12.  What    kinds  of  voltameters  are  most  commonly   used  for   the  calibration    of 
instruments? 


X. 

MEASUREMENT  OF  ELECTRICAL  RESISTANCE. 

All  useful  methods  of  measuring  electrical  resistance  depend 
directly  upon  the  indications  of  Ohm's  Law.  The  simplest  method 
of  measuring  a  resistance  is  by  what  is  called  substitution.  The 
resistance  to  be  measured  is  connected  in  series  with  a  galvanometer 
and  a  constant  battery  and  the  deflection  of  the  galvanometer  is 
noted.  Then  the  unknown  resistance  is  removed  from  the  circuit 
and  a  variable  resistance  box  or  rheostat  (Fig.  44)  is  substituted  for  it. 
The  resistance  of  the  resistance  box  is  then  adjusted  until  the  galvan- 
ometer deflection  is  the  same  as  before.  Then  the  resistance  in- 
serted in  the  circuit  by  means  of  the  box  is  equal  to  the  unknown 
resistance,  because  in  the  two  cases  the  same  current,  as  shown  by 
the  galvanometer,  flows  through  the  circuit,  and  the  total  electrical 
pressure  acting  in  the  circuit  is  the  same  in  each  case;  consequently, 
according  to  Ohm's  Law,  the  resistance  of  the  total  circuit  must  be 
the  same  in  the  two  cases.  It  is  necessary  that  no  changes  be  made 
in  the  circuit  besides  the  substitution  of  the  variable  known  resist- 
ance for  the  unknown  one. 

Resistance  boxes  are  generally  boxes  containing  spools  of  silk- 
covered  wire,  each  of  known  resistance,  which  may  be  used  in  elec- 
trical measurements.  German  silver  or  some  similar  alloy  having  a 
comparatively  low  conductivity  and  a  small  temperature  co-efficient 
(Lesson  7,  page  44,  and  Lesson  7,  page  49,)  is  generally  used  in 
making  the  spools  or  coils  for  resistance  boxes.  In  making  the  coils, 


FIG.  44. 


FIG.  45. 


the  proper  length  of  wire  for  each  is  taken  and  doubled  at  the  middle, 
and  is  then  wound  double  upon  a  spool.  The  object  of  doubling  the 
wire  is  to  avoid  the  effects  due  to  self-inductance,  which  will  be 
explained  later.  After  the  spools  are  wound,  they  are  dipped  in 
paraffine  and  then  placed  inside  the  box  and  fastened  to  the  under 
side  of  the  top  of  the  box  by  brass  bolts  (Fig.  45),  which  also  fasten 
brass  blocks  to  the  upper  side  of  the  top.  (Fig.  44.)  The  individ- 
ual ends  of  each  coil  are  connected  to  adjoining  brass  blocks  so  that 
all  the  coils  are  in  series  when  the  blocks  are  not  connected.  This 
is  shown  in  Fig.  46,  where  the  ends  a  and  b  of  one  of  the  resistance 
coils  are  fastened  to  the  brass  blocks  E  and  H,  while  the  ends  c  and 
d  of  the  next  coil  are  fastened  to  the  blocks  H  and  M.  The  brass 
blocks  are  so  arranged  that  they  may  be  connected  together  by  plugs 
which  fit  in  tapering  holes  as  shown  in  the  figure. 

If  such  a  resistance  box  be  connected  in  a  circuit  when  all  the 
plugs  are  removed,  the  current  flows  through  all  the  resistance  coils 
in  series.  If  one  of  the  plugs  be  inserted  in  a  hole,  the  correspond- 
ing resistance  coil  is  short-circuited — that  is,  a  negligibly  small 
resistance  (that  of  the  plug)  is  connected  in  parallel  with  it,  and  no 
appreciable  current  flows  through  the  coil.  Since  the  resistance  of 
the  plug  is  practically  negligible  the  resistance  of  the  circuit  is 
reduced  by  the  amount  of  the  resistance  of  the  corresponding  coil 
when  a  plug  is  inserted.  The  resistance  of  a  box  may  therefore  be 
varied  at  will  by  simply  inserting  or  removing  plugs. 

Resistance  boxes  generally  have  a  series  of  coils  of  different 
resistances, usually  given  in  tenths,  units,  tens,  hundreds, etc., of  ohms. 
The  final  adjustment  of  the  resistance  of  the  coils  of  a  fine  resistance 
box  is  a  matter  requiring  rreat  care,  and  is  effected  by  soldering  more 
or  less  of  the  doubled  ends  of  the  wire  together  after  the  spool  is 
mounted  in  its  box.  In  order  that  the  adjustment  may  be  made  in 
this  way  it  is  necessary  that  the  resistance  of  the  coil  when  wound 


67 


on  the  spool  be  a  little  greater  than  the  desired  final  value.  When 
adjusting  coils  great  care  must  be  taken  to  avoid  errors  due  to  the 
temperature  of  the  coils  changing,  since  the  wires  are  likely  to  be- 
come heated  by  the  soldering. 

The  measurements  for  determining  the  exact  value  of  the  coils 
are  made  by  what  is  called  a  Wheatstone  bridge,  after  Wheatstone, 
an  English  scientist  and  inventor.  This  consists  of  an  arrangement 
of  resistance  coils  which  are  used  with  a  battery  and  galvanometer  as 
shown  diagramatically  in  Fig.  47.  In  the  figure,  A,  B  and  C 
represent  resistance  boxes  with  coils  of  known  resistance ;  D  is  the 
resistance  to  be  measured;  L  and  G  are  a  battery  and  a  galvanometer; 
K!  and  K2  are  keys  placed  in  the  circuits  with  the  battery  and  galvan- 
ometer, by  means  of  which  the  circuits  may  be  made  and  broken; 
M,  N,  P,  and  Q  are  points  where  the  various  bridge  circuits  are  con- 
nected together. 

From  an  application  of  the  law  of  the  fall  of  potential  along  a 
resistance  as  deduced  from  Ohm's  Law  (Lesson  7,  page  49),  it  is  easy 
to  see  how  the  resistance  of  the  coil  D  is  determined  by  this  device. 
Suppose  the  battery  key,  Kj,  be  depressed,  then  current  will  flow 
from  the  battery  through  the  key  to  the  point  P.  Here  it  divides, 
and  part  goes  to  Q  by  way  of  M  and  the  other  part  by  way  of  N. 
From  Q  the  current  returns  to  the  battery.  The  points  P  and  Q  are 
at  a  certain  difference  of  electrical  pressure  which  depends  upon  the 
battery,  and  which  we  will  call  B,  and  the  fall  of  pressure  from  P 
to  Q  by  way  of  either  M  or  N  is  equal  to  E.  The  fall  of  pressure 
between  P  and  M  is  (according  to  the  law  that  the  fall  of  pressure  is 
proportional  to  the  resistance  passed  over)  B^  where  d  and  c  repre- 
sent the  resistances  of  the  branches  of  the  bridge  D  and  C  respect- 
ively. In  the  same  way  the  fall  of  pressure  between  P  and  N  is  equal 
to  B^.  If  the  fall  of  pressure  between  P  and  M  is  greater  than  that 
between  P  and  N,  the  point  M  is  at  a  lower  pressure  than  N,  and  if 


FIG.  48. 


the  galvanometer  key  be  depressed  a  current  will  flow  from  N  to  M 
through,  the  galvanometer,  deflecting  the  needle.  Now  if  the  resist- 
ance b  be  increased  until  the  fall  of  pressure  between  P  and  N  is  the 
greater,  a  current  will  flow  from  M  to  N  when  the  galvanometer  key 
is  depressed  and  the  needle  will  be  deflected  in  the  opposite  direction. 
Finally  if  the  resistance  of  b  be  so  adjusted,  by  arranging  the  plugs, 
that  the  fall  of  pressure  between  P  and  M  and  between  P  and  N  is 
the  same,  the  pressures  at  the  points  M  and  N  are  equal  and  no  cur- 
rent will  flow  through  the  galvanometer  when  the  key  is  depressed, 
and  the  needle  will  not  be  deflected.  The  bridge  is  then  said  to  be 
balanced.  In  this  case  E^  =  B^,  or,  what  is  the  same  thing,  \  =  ~- 
From  this  proportion  we  get  d=b^;  that  is,  the  unknown  resistance 
of  D  is  equal  to  the  resistance  of  B,  multiplied  by  the  resistance  of 
C  divided  by  that  of  A.  Put  in  the  form  of  a  proportion  this  may 
be  written,  a  is  to  b  as  c  is  to  d\  or,  a  is  to  c  as  b  is  to  d.  The  solution 
of  either  of  these  proportions  gives  the  results  given  above.  If  a  and 
c  are  equal,  and  the  bridge  is  balanced,  b  must  be  equal  to  d,  so  that 
the  resistance  of  the  unknown  branch  or  arm  of  the  bridge  is  given 
at  once  by  the  resistance  of  the  coils  in  circuit  at  B.  In  the  figure, 
the  resistance  of  C  is  ten  times  as  great  as  that  of  A  and  therefore  the 
resistance  of  D  is  ten  times  that  of  B  and  is  150  ohms.  The  arms  A 
and  C  are  generally  called  the  ratio  arms  of  the  bridge  and  B  the 
rheostat. 

The  Wheatstone  bridges  that  are  commonly  used  are  not  made 
up    from   three  separate  resistance    boxes  as  indicated  in   Fig.   47. 


FIG.  49. 

The  common  forms  of  Wheatstone  bridge  contain  all  the  resistance 
coils  in  one  box,  and  the  coils  are  connected  up  in  such  a  way  that 
they  form  a  bridge.  Binding  posts,  generally  marked  B,  G,  and  R  or 
X,  are  arranged  for  the  connection  to  the  bridge  at  the  proper  points 
of  the  battery,  galvanometer  and  the  unknown  resistance  which  is  to 
be  measured.  Fig.  48  shows  such  a  bridge  made  up  in  a  box  so  as  to 
be  portable.  At  the  front  are  seen  the  battery  and  galvanometer 
keys.  This  form  of  bridge  is  often  called  the  postoffice  pattern,  be- 
cause its  arrangement  is  similar  to  the  bridge  used  by  the  British  de- 
partment of  postal  telegraphs.  Fig.  49  shows  another  way  in 
which  the  resistance  coils  are  often  arranged  to  make  a  very  accu- 
rate and  convenient  bridge  for  use  in  laboratories  where  it  may  be 
permanently  fixed. 

Measurements  of  resistance  may  be  made  with  a  fine  bridge  to  a 
remarkable  degree  of  accuracy.  In  fact,  the  ease  and  accuracy  to  be 
attained  in  bridge  measurements  are  only  rivalled  in  weighing  with 
fine  balances.  It  is  not  unusual  to  have  the  resistance  coils  of  a  fine 
bridge  adjusted  to  an  error  of  less  than  1-50  of  one  per  cent  of  their 
desired  value  as  represented  by  a  standard  coil,  or  within  two  parts 
out  of  ten  thousand  at  a  fixed  temperature.  In  adjusting  the  coils  of  a 
resistance  box  so  closely,  or  in  accurately  measuring  a  resistance  by  a 
bridge,  careful  account  must  be  taken  of  the  temperature.  If  the  re- 
sistance coils  of  a  bridge  are  exactly  correct  at  one  temperature  they 
are  not  correct  at  any  other  temperature.  (Lesson  7,  page  49!) 

It  is  frequently  convenient  to  have  a  portable  bridge  which  is 
entirely  self  contained — that  is,  the  box  of  which  contains  the  gal- 
vanometer and  battery  as  well  as  the  resistance  coils.  In  this  case 
all  that  is  necessary  to  make  a  measurement  of  resistance  is  to  connect 
the  unknown  resistance  to  the  proper  binding  posts,  press  the  keys, 
and  adjust  the  plugs  till  the  galvanometer  gives  no  deflection.  Such 
bridges  are  generally  called  testing  sets.  One  is  shown  in  Fig.  50. 

In  making  resistance  measurements  with  a  bridge  the  battery 
key  should  be  depressed  before  the  galvanometer  key,  or  irregular 
and  incorrect  indications  will  often  be  given  on  account  of  the  self- 
inductance  of  the  unknown  resistance.  This  is  particularly  true 
when  the  unknown  resistance  is  the  windings  of  an  electromagnet 


70 


FIG.  50. 


FIG.  51. 

or  any  of  the  windings  of  a  dynamo.  Great  care  should  always  be 
exercised  not  to  injure  the  galvanometer  or  the  fine  wire  coils  by 
passing  too  great  a  current  through  them. 

For  very  accurate  comparisons  of  two  resistances,  as  when  the 
value  of  a  standard  resistance  coil  is  to  be  determined  in  terms  of  a 
mercury  column  or  another  coil,  the  Wheatstone  bridge  is  made  up 
in  another  form.  (Fig.  51.)  Here  we  have  two  arms  of  the 
bridge,  A  and  B,  made  up  of  a  uniform  wire  of  high  resistance  and 
small  temperature  co-efficient.  The  other  two  arms  contain  the  two 
coils.  The  galvanometer  terminal  corresponding  to  M  (Fig.  47) 
is  made  up  by  means  of  a  binding  post,  bat  the  other  terminal  is 


71 


arranged  so  that  contact  may  be  made  at  any  point  along  the  bridge 
ivire.  When  the  galvanometer  contact  is  placed  at  the  point  on  the 
bridge  wire  which  gives  a  balance,  the  resistances  of  the  parts  of  the 
bridge  wire  on  each  side  of  the  galvanometer  contact  are  to  each 
other  as  the  two  resistance  coils,  according  to  the  bridge  formula 
already  developed.  When  the  bridge  wire  is  calibrated,  that  is,  when 
the  resistance  per  centimeter  of  length  at  every  point  of  the  wire  is 
determined,  the  ratio  of  the  resistance  of  the  two  coils  is  given  by  the 
ratio  of  the  resistances  of  the  two  parts  of  the  wire.  When  the  bridge 
wire  is  very  uniform  and  the  measurement  is  not  required  to  be  very 
exact,  the  resistances  of  the  two  parts  of  the  wire  may  be  taken  to  be 
proportional  to  their  lengths.  Bridge  wires  are  usually  made  of  an 
alloy  containing  platinum  and  silver,  or  platinum  and  iridium.  Bridges 
of  this  form  are  usually  called  divided  wire  or  meter  bridges. 

Measurements  of  very  great  resistances,  such  as  the  insulation 
reistance  of  a  well-insulated  wire  between  its  conductor  and  ground, 
often  require  a  higher  power  than  may  be  conveniently  reached  by  a 
bridge.  In  this  case  a  fine  reflecting  galvanometer  and  a  large  test- 
ing battery  are  used.  The  testing  battery  usually  consists  of  silver 
chloride  cells  put  up  in  sets  of  50  or  100  in  boxes  so  as  to  be  porta- 
ble. The  galvanometer  and  battery  are  connected  in  series  with 
some  known  large  resistance,  and  the  deflection  of  the  galvanometer 
is  read.  Then  the  known  resistance  is  removed  from  the  circuit  and 
that  which  it  is  desired  to  measure  is  inserted  in  its  place.  The 
deflection  of  the  galvanometer  is  again  read  and  from  the  two  deflec- 
tions the  unknown  resistance  may  be  calculated.  The  known  or 
standard  resistance'^  usually  from  25,000  to  1,000,000  ohms  in  resist- 
ance, i, 000,000  ohms  is  called  a  megohm,  the  prefix  "meg"  coming 
from  a  Greek  word  meaning  great.  The  insulation  resistances  of 
wires  and  cables  that  are  measured  thus  are  frequently  as  great  as 
thousands  of  megohms,  so  that  it  is  necessary  to  use  a  very  fine  gal- 
vanometer to  get  a  readable  deflection  through  them,  and  the  gal- 
vanometer must  be  shunted  (Lesson  7,  page  48)  when  the  deflec- 
tion is  taken  with  the  standard  resistance  in  circuit.  Galvanometers 
usually  have  corresponding  shunt  boxes  come  with  them  which  have 
three  coils  marked  respectively  i,  gV,  -99-9-  When  the  shunt  box  is 
connected  in  parallel  with  the  galvanometer,  either  of  these  shunts 
may  be  placed  in  the  circuit  by  means  of  a  plug,  or  the  shunt  circuit 
may  be  broken.  When  the  shunts  are  plugged  into  circurt,  TV,  TOD> 
or  TW^  Part  of  the  whole  current  flows  respectively  through  the  gal- 
vanometer. Fig.  52  shows  a  common  form  of  shunt  box. 

As  an  example,  suppose  it  is  desired  to  measure  the  insulation  of 
an  electric  light  cable  one-half  mile  long,  a  fine  galvanometer,  a  test- 
ing battery  of  200  cells,  and  a  standard  resistance  of  one-half  megohm 
being  available.  When  connected  up  and  shunted  by  the  -g-J-g-  shunt, 


the  galvanometer  gives  a  deflection  of  one  hundred.  Then  its  con- 
stant, or  the  resistance  of  the  circuit  in  megohms  which  would  be 
indicated  by  a  deflection  of  i  when  the  galvanometer  is  not  shunted, 
is  loox  1000  x^  =  5OOOO,  1000  being  the  multiplying  power  of  the 
shunt  and  y2  the  value  of  the  standard-resistance  in  megohms.  Now 
when  the  standard  resistance  is  removed  from  the  circuit,  and  in  its 
place  one  end  of  the  connecting  wire  is  attached  to  the  conductor  ot 
the  cable  and  the  other  end  to  the  ground,  suppose  the  reading  of  the 
galvanometer  without  a  shunt  is  50.  The  insulation  resistance  of  the 
cable  is  -5-°A°A  =  1000  megohms.  Then  the  insulation  resistance  of  a 
similar  cable  for  a  length  of  one  mile  is  500  megohms,  since  the 
paths  for  the  current  to  leak  out  of  the  two  half  miles  are  in  parallel. 
Other  methods  of  measuring  high  resistances  and  special  methods 
of  measuring  very  low  resistances  are  sometimes  used  but  they  need 
not  receive  attention  here. 

Copyrighted,  1894, 


The  National  School  of  Electricity. 

REVIEW   OF   LESSON  X. 

Points  for  Review:     1.     How  is  resistance  measured  by  substitution? 

2.  How  are  resistance  boxes  generally  made? 

3.  Why  is  German  silver  usually  used  in  resistance  boxes? 

4.  How  are  the  coils  of  resistance  boxes  adjusted? 

5.  What  is  a  Wheatstone  bridge? 

6..    What  is  the  process  of  measuring  a  resistance  by  a  Wheatstone  bridge? 

7.  Suppose  a  resistance  is  to  be  measured  by  bridge,  and  after  the  bridge  is  balanced 
the  rheostat  resistance  reads  15.6  ohms,  while  the  ratio  arms  are  100  (A)  to  10  (C),  what 
is  the  value  of  the  unknown  resistance? 

8.  In  another  case  suppose  the  rheostat  reads  2,600  ohms  and  the  ratio  arms  are  10 
(A)  to  1,000  (C),  what  is  the  value  of  the  unknown  resistance? 

9.  Why  should   the  battery   key  be  depressed  before   the  galvanometer  key  when 
making  bridge  measurements? 

10.  How  may  very  high  resistances  be  measured? 

11.  Suppose  a  reflecting  galvanometer  shunted  with  the    §^9  shunt    gives    a  deflec- 
tion of  80  when  using  a  certain  battery,  the  standard  resistance  being  25,000  ohms;  what 
is  the  galvanometer  constant? 

12.  Suppose  the  deflection   is  120  with   the   \  shunt  when   a  certain    resistance  is 
substituted  for  the  standard  as  above,  other  things  being  unchanged,  what  is  the  value  of 
the  resistance? 


XI. 

EVERY    DAY    MEASUREMENTS   OF    ELECTRIC 
CURRENTS  AND  PRESSURES. 

We  have  already  seen  that  electric  currents  may  be  measured 
by  taking  advantage  of  three  different  and  independent  effects  of  the 
current.  These  are:  i,  the  electrochemical  effect;  2,  the  magnetic 
effect;  3,  the  heating  effect.  By  taking  advantage  of  the  first  effect 
we  measure  currents  by  voltameters  (Lesson  9,  page  62);  as  a  result 
of  the  second  effect  we  measure  currents  by  means  of  galvanometers 
(Lesson  9,  page  58);  from  the  third  effect  we  may  measure  currents 
by  means  of  the  expansion  of  a  wire  which  is  heated  by  the  passage 
of  the  current  through  it  (Lesson  8,  page  54). 

Voltameters,  as  already  said,  are  principally  used  for  calibrating 
galvanometers  or  for  similar  purposes,  as  they  are  not  sufficiently 
convenient  for  general  use.  The  liquid  must  be  kept  fairly  pure  and 
of  the  proper  density.  Conveniences  must  be  available  for  cleaning, 
drying,  and  accurately  weighing  the  cathodes.  In  order  that  a  satis- 
factory measurement  of  the  current  may  be  made,  the  period  during 
which  it  flows  through  a  voltameter  must  be  considerable. 

74 


For  one  purpose  only  have  they  been  found  particularly  useful 
in  everyday  measurements;  that  is,  as  a  meter  such  as  many  of  the 
Edison  Illuminating  Companies  use  (Lesson  9,  page  64) .  Volta- 
meters were  used  for  this  purpose  in  the  early  days  of  electric  light- 
ing with  incandescent  lamps  and  have  continued  in  use  until  now. 
Even  for  that  purpose  their  everyday  use  is  not  being  extended,  as 
good  mechanical  meters  that  are  more  reliable  are  now  to  be  had. 

Nearly  all  our  common  instruments  for  measuring  currents  de- 
pend upon  the  magnetic  effect  of  the  current  for  their  indications, 
and  are  really  modified  galvanometers  with  pointers  to  show  the 
deflection. 

Galvanometers  or  other  instruments  intended  especially  for  con- 
venient use  in  every  day  measurements  of  currents,  are  generally 
called  amperemeters  or  ammeters,  because  they  measure  amperes. 
Amperemeters  are  made  in  various  forms,  all  more  or  less  portable. 
Almost  every  manufacturer  of  dynamos,  or  other  electrical  machinery, 
manufactures  amperemeters  which  may  be  used  in  service  with 
their  machines.  Amperemeters  are  used  universally  where  electric- 
ity is  used,  and  they  are  made  to  measure  currents  consisting  of  only 
a  few  thousandths  of  an  ampere,  or  milliamperes  (milli  comes  from  a 
Latin  word  meaning  thousand),  up  to  the  enormous  currents  gener- 
ated by  some  of  the  larger  electric  lighting  plants  consisting  of 
thousands  of  amperes.  In  large  electric  lighting  plants  or  works 
many  amperemeters  may  be  seen  mounted  on  the  wall  or  on  a  board 
among  switches  for  controlling  the  current.  These  are  used  to  show 
the  dynamo  attendants  how  much  current  is  being  generated  by  the 
plant  at  any  moment,  and  what  proportion  is  furnished  by  each 
dynamo.  Amperemeters  are  used  in  laboratories  to  determine  the 
current  used  in  experiments,  and  to  determine  the  amount  of  current 
used  in  the  operation  of  electric  lamps,  electric  motors,  or  other 
electric  devices.  Physicians  use  amperemeters  to  measure  the  cur- 
rents used  in  the  electrical  treatment  of  their  patients.  For  the  latter 
purpose  the  currents  are  usually  measured  in  milliamperes.  The 
currents  used  in  telegraphy  are  also  usually  measured  in  milliamperes, 
and  the  currents  used  in  operating  telephones  are  usually  measured 
in  microamperes,  or  millionths  of  amperes  (micro  coming  from  a 
Greek  word  meaning  small).  Amperemeters  that  are  specially  made 
to  measure  thousandths  of  amperes,  or  milliamperes,  are  called 
milliamperemeters.  Externally,  milliamperemeters  look  like  ordi- 
nary amperemeters,  to  which  they  bear  the  same  relation  that  a  very 
sensitive  galvanometer  bears  to  a  similar  but  less  sensitive  instru- 
ment. 

The  mechanical  details  entering  into  the  construction  of  mag- 
netic amperemeters  differ  very  widely.  They  may  be  roughly 
divided  into  three  classes:  (i)  those  having  permanently  magnetized 
parts  which  are  moved  by  magnetic  force  set  up  by  a  current  in  the 


1i 
EDISON 

SYSTEM, 

AMPERE  METER 


FIG.  53 


FIG.  56. 


_ _ ^  FIG.  57. 

FIG.  54. 

coils  of  the  instrument;  (2)  those  having  soft  iron  parts  which  are 
moved  by  the  magnetic  attraction  set  up  by  a  current  in  the  coils  of 
the  instrument;  (3)  those  having  no  iron  in  their  construction,  but 
having  two  coils,  one  of  which  is  moved  by  magnetic  force  exerted 
between  them  when  a  current  flows  in  both.  The  moving  parts  of 
amperemeters  are  -usually  mounted  on  pivots  made  so  that  the 
friction  is  small.  If  the  magnetic  force  caused  by  a  current  in  the 
coils  had  nothing  except  the  friction  to  overcome,  every  current 
would  pull  the  pointer  clear  across  the  scale  to  the  stop.  It  is  desir- 
able to  construct  the  instrument  so  that  the  movement  of  the  pointer 
is  proportional  to  the  current  in  the  coil,  so  a  proper  force  must  be 
arranged  to  hold  the  pointer  back.  This  may  be  done  by  properly 
counter  weighting  the  moving  parts  so  that  the  magnetic  force  must 
raise  them  against  the  force  of  gravity,  or  by  arranging  a  proper 
spring  to  oppose  the  magnetic  force.  Fig.  53  shows  an  instrument 
in  which  a  curved  iron  wire  is  drawn  into  a  coil  of  wire  where  the 


76 


FIG   55. 

current  flows  through  the  coil.  The  weight  of  the  moving  parts  of 
the  instrument  serves  to  keep  the  pointer  at  zero  when  no  current 
flows.  When  a  current  flows  it  exerts  an  attraction  on  the  iron  wire 
core,  which  overcomes  the  effect  of  the  weight  of  the  moving  parts,  the 
iron  core  is  attracted  into  the  coil  a  certain  distance,  and  the  pointer 
moves  proportionally.  This  instrument  evidently  belongs  to  the  sec- 
ond class.  Instruments  of  the  second  class  may  be  cheaply  made. 
They  are  therefore  commonly  made  by  dynamo  builders  for  use  with 
their  dynamos  in  electric  light  plants'.  Fig.  54  shows  another 
form  of  amperemeter  of  the  same  class. 

Instruments  having  soft  iron  in  their  moving  parts  cannot  be 
made  extremely  accurate  because  the  iron  does  not  always  respond 
equally  to  the  same  magnetic  changes  on  account  of  its  coercive 
force  (L,esson  5,  page  31);  consequently  instruments  of  the  second 
class  can  only  be  used  where  great  accuracy  is  not  required.  It  is 
sufficient  for  the  amperemeters  used  in  electric  plants  to  be  correct 
within  five  per  cent,  and  instruments  of  the  second  class  serve 
jvery  well.  For  testing  which  requires  greater  accuracy  instruments 
belonging  to  the  first  and  third  classes  must  be  used.  These  can  be 
made  so  that  their  readings  do  not  vary  more  than  one-half  of  one 
per  cent  from  true  values  when  they  are  used  with  proper  care. 

Fig.  55  shows  a  Weston  amperemeter,  which  is  practically  a 
d'Arsonval  galvanometer  with  the  moving  coil  mounted  on  pivots 
and  arranged  with  a  pointer  to  play  over  a  scale,  and  the  whole  ar- 
ranged in  a  very  convenient  portable  form.  This  instrument  may 
be  looked  upon  as  the  most  satisfactory  representative  of  the  first 
class,  to  which  it  bears  the  same  relation  that  a  d'Arsonval  galvano- 
meter bears  to  a  galvanometer  with  a  movable  magnetic  needle. 


Weston  amperemeters  are  used  a  great  deal  where  accurate  portable 
current  measuring  instruments  are  required,  because  they  are  accu- 
rate, convenient,  and  well  made. 

Magnetic  instruments  belonging  to  the  third  class  are  really  not 
galvanometers,  but  are  called  electrodynamometers,  because  their  in- 
dications are  caused  by  the  magnetic  pull  of  the  current  in  the  fixed 
and  movable  coils  upon  itself.  Fig.  56  shows  the  ordinary  form  of 
electrodynamometer  when  arranged  for  use  as  an  amperemeter.  This 
is  often  called  the  Siemens  electrodynamometer.  In  it,  one  coil  is  fast- 
ened to  the  frame  of  the  instrument,  and  the  other,  which  stands  at 
right  angles  to  the  first,  is  suspended  by  a  heavy  silk  fibre  so  that  it 
is  free  to  move.  The  end,  of  the  wire  composing  the  movable  coil 
dips  into  little  cups  containing  mercury  which  are  connected  with  a 
circuit  so  that  the  current  can  enter  and  leave  the  coil.  The  mov- 
able coil  is  attached  to  a  spring,  the  other  end  of  which  is  connected 
to  a  thumbscrew  by  means  of  which  the  spring  may  be  twisted.  When 
a  current  flows  in  the  coil,  the  magnetic  force  tends  to  turn  the 
movable  coil  around  so  as  to  place  it  parallel  with  the  fixed  coil. 
(Lesson  6,  page  38.)  This  force  is  balanced  by  twisting  the  spring 
by  means  of  the  thumbscrew.  The  amount  of  twist  as  shown  by  a 
pointer  attached  to  the  screw  is  proportional  to  the  force  exerted  by  the 
coils  on  each  other.  This  force  is  proportional  to  the  square  of  the 
current  flowing  in  the  coil,  since  the  magnetism  set  up  by  each  coil 
is  proportional  to  the  current  and  they  act  on  each  other  mutually. 

Other  instruments  for  measuring  currents  by  their  direct  mag- 
netic action,  as  in  the  Siemens  electrodynamometer,  have  been 
designed,  but  they  have  not  been  made  sufficiently  portable  to  bring 
them  into  much  use.  The  most  important  of  these  are  the  current 
balances  of  Sir  William  Thomson,  now  Lord  Kelvin.  In  these  the 
fixed  and  movable  coils  are  parallel  and  horizontal.  The  force  with 
which  the  coils  tend  to  move  toward  each  other  when  a  current  flows 
in  them  is  directly  balanced  and  weighed  by  means  of  a  slider  mov- 
ing on  a  scale  beam.  In  order  to  avoid  any  effect  from  the  earth's 
magnetism,  coils  are  placed  at  both  ends  of  the  balance  arm  and  are 
electrically  connected  so  that  the  magnetic  force  of  the  two  sets  of 
coils  tends  to  tip  the  beam  in  the  same  direction. 

Instruments  utilizing  the  heating  effect  of  the  current  may  be 
called  hot  wire  instruments.  If  the  wire  be  carefully  enclosed  so  that 
its  temperature  is  not  affected  by  air  currents,  it  will  rise  to  a  definite 
number  of  degrees  in  temperature  for  every  current  that  is  passed 
through  it,  and  the  rise  is  proportional  to  the  square  of  the  current. 
(Lesson  8,  page  53.)  The  length  of  the  wire  increases  practically  in 
direct  proportion  to  its  rise  in  temperature  when  it  is  heated,  and  the 
length  again  decreases  when  the  wire  is  cooled.  Consequently,  when 
currents  of  different  strength  flow  through  a  wire  it  will  take  up  a 
corresponding  length  with  each  current,  and  measuring  its  length 
therefore  measures  the  square  of  the  current.  A  simple  form  of 


amperemeter  depending  on  this  action  is  shown  in  Fig.  57.  A  long 
thin  wire  is  clasped  at  one  end  in  a  stationary  binding  post  and  the 
other  end  is  wrapped  around  and  fastened  to  a  small  wheel  of  metal. 
This  wheel  is  supported  in  steel  pivots,  one  of  which  is  connected  to 
another  binding  post.  Tlie  wire  is  kept  under  a  constant  strain  by 
means  of  a  spring  the  end  of  which  is  also  fastened  to  the  periphery 
of  the  wheel,  so  that  when  the  wire  is  heated  and  lengthens,  the 
wheel  is  turned  by  the  contraction  of  the  spring,  and  when  the  wire 
is  again  cooled  and  contracts  it  pulls  the  wheel  back  to  its  old  posi- 
tion. The  wheel  carries  a  pointer  the  position  of  which  may  be  read 
on  the  graduated  scale  when  any  current  flows  in  the  wire. 

Many  amperemeters  have  scales  that  are  uniformly  graduated 
and  the  readings  of  which  can  only  be  converted  into  amperes  by 
consulting  a  calibration  curve  or  a  table  giving  the  values  of  differ- 
ent readings  in  amperes.  In  other  instruments  multiplying  the  read- 
ings by  a  fixed  constant  which  has  been  experimentally  determined, 
converts  them  into  amperes.  In  still  other  instruments,  which  are 
said  to  be  direct  reading,  the  scales  are  so  divided  and  marked  that 
the  divisions  read  directly  in  amperes.  It  is  needless  to  say  that 
direct  reading  instruments  are  the  most  convenient  for  use. 

Currents  which  rapidly  alternate  in  direction,  as  do  the  currents 
of  many  electric  light  plants,  cannot  be  measured  by  magnetic 
instruments  having  permanent  magnets,  since  the  tendency  of  such 
currents  is  to  first  deflect  the  moving  parts  in  one  direction  and  then 
in  the  other,  and  the  pointer  stands  still  or  nearly  so.  Such  currents 
can  be  measured  by  magnetic  instruments  of  the  second  class  because 
the  soft  iron  core  is  a/ways  attracted  by  a  coil  in  which  a  current  flows 
without  regard  to  the  direction  of  the  current.  The  iron  cores  in 
instruments  designed  to  measure  alternating  currents  must  be  made 
up  from  fine  iron  wires  so  that  currents  shall  not  be  set  up  in  them 
by  the  reversals  of  the  magnetism,  as  will  be  explained  later. 

Klectrodynamometers  and  other  instruments  depending  for  their 
indications  upon  the  mutual  attractions  of  two  coils,  may  be  used 
to  measure  alternating  currents  because  the  current  reverses  in  the 
two  coils  at  the  same  instant,  and  the  magnetic  attraction  between 
the  coils  is  therefore  always  in  the  same  direction.  The  heating 
effect  of  currents  is  always  independent  of  their  direction,  so  that 
hot  wire  instruments  may  be  used  to  measure  alternating  currents. 

When  very  large  currents  are  to  be  measured,  it  is  often  incon- 
venient and  expensive  to  build  an  amperemeter  of  sufficient  capacity 
for  the  purpose.  In  this  case  an  amperemeter  of  small  capacity  may 
be  shunted  by  a  copper  or  german  silver  wire  or  rod,  and  the 
shunted  instrument  may  then  be  calibrated  and  used  to  measure  the 
large  current.  This  arrangement  is  becoming  quite  common  in  the 
largest  electric  light  works  where  very  great  currents  are  to  be 
measured.  Nearly  all  Weston  amperemeters  consist  of  a  milliam- 
peremeter  arranged  with  a  proper  shunt  inside  the  case  so  that  the 
desired  range  is  obtained. 

79 


FIG.  58. 

The  commonest  method  of  measuring-  an  electric  pressure  is  to 
measure  the  current  which  it  causes  to  pass  through  a  known  high 
resistance.  The  resistance  may  be  connected  permanently  in  the 
circuit  of  a  sensitive  amperemeter,  such  as  a  milliamperemeter,  and 
the  instrument  may  be  calibrated  so  that  its  indications  may  be 
readily  converted  into  volts. 

Instruments  that  are  used  for  every-day  measurements  of 
electric  pressures  are  called  voltmeters,  because  they  measure  volts. 
By  properly  dividing  the  scale  upon  which  the  indications  are  made, 
voltmeters  may  be  made  direct  reading.  Fig.  58  shows  a  Weston 
direct  reading  voltmeter,  in  which  the  working  parts  are  similar  to 
those  of  the  amperemeter  shown  in  Fig.  55;  but  in  the  voltmeter, 
a  high  resistance  spool  of  fine  wire  is  placed  in  series  with  the 
d'Arsonval  galvanometer  coils,  instead  of  a  low  resistance  s^*"it 
being  placed  in  parallel  with  it,  as  is  done  in  the  amperemeter.  A 
voltmeter  is  shown  in  Fig.  59  which  is  made  upon  the  same 
principle  as  the  amperemeter  shown  in  Fig.  53,  but  the  coil  is 
wound  with  many  turns  of  fine  wire,  making  a  high  resistance, 
instead  of  being  made  with  a  few  turns  of  coarse  wire.  This  form 
of  voltmeter  has  the  same  fault  as  the  amperemeter  of  the  same  class, 
that  of  being  not  very  accurate,  and  it  therefore  is  not  as  satisfactory 
for  use  in  many  places  as  more  accurate  instruments  made  with  very 
little  or  no  iron  in  their  working  parts.  In  electric  light  plants 
where  current  is  produced  for  use  in  incandescent  lamps,  it  is  very 
important  that  the  pressure  be  kept  as  closely  as  possible  to  the 
exact  pressure  with  which  the  lamps  were  designed  to  be  used. 
Consequently,  in  such  places  the  most  accurate  and  reliable  volt- 
meters or  pressure  indicators,  as  they  are  sometimes  called,  are 
needed. 

Voltmeters  of  this  kind  are  usually  made  with  a  very  high 
resistance  so  that  only  a  small  current  flows  through  them  and  they 

80 


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FIG.  63. 


FIG.  61. 


SI 


XJNIVE 


may  therefore  be  used  without  an  appreciable  change  of  the  current 
in  a  circuit. 

Fig.  60  shows  a  hot  wire  voltmeter  which  is  called  after  its 
inventor,  Cardew.  This  was  at  one  time  largely  used  to  measure 
alternating  electric  pressures,  and  is  still  used  quite  generally  for  the 
same  purpose  in  England.  The  indications  of  this  instrument  are 
dependent  upon  the  expansion  of  a  very  fine  platinum-silver  wire 
(YO^-O  inch  in  diameter)  through  which  the  current  passes.  This  wire 
is  from  8  to  12  feet  long  and  of  such  high  resistance  per  foot  that 
its  resistdnce  alone  is  sufficient  for  use  up  to  a  pressure  of  1 20  volts, 
but  another  resistance  coil  is  put  in  series  with  the  instrument  when 
it  is  used  to  measure  higher  pressures. 

Another  entirely  distinct  method  of  measuring  electric  pres- 
sures is  by  means  of  electrometers.  On  page  8  of  L,esson  i,  it  was 
said  that  electrometers  are  instruments  for  determining  the  amount 
of  electricity  on  a  charged  body  by  measuring  its  attraction  for  an- 
other charged  body.  It  w^also  explained  on  page  13  of  Lesson  2  that 
electricity  at  rest  at  a  high  pressure  constitutes  a  positive  charge, 
and  electricity  at  rest  at  a  low,pressure  constitutes  a  negative  charge. 
It  is  a  fact  that  the  terms  positive  and  negative  charge  must  be  taken 
as  relative  terms  exactly  as  are  the  terms  high  and  low  pressure.  An 
electrometer  is  an  instrument  by  means  of  which  the  attraction 
between  two  charges  may  be  measured.  One  form  of  electrometer  is 
shown  in  Fig.  61.  In  this  there  is  a  needle  made  of  aluminum  and 
a  sort  of  pillbox  cut  into  qiiadrants  (quarters).  If  the  opposite  quar- 
ters be  connected  together  as  shown  and  one  pair  of  quarters  be  con- 
nected to  the  needle,  and  a  charge  of  one  sign  be  communicated  to 
the  needle  and  its  connected  pair  of  quadrants,  and  a  charge  of  the 
opposite  sign  to  the  other  pair  of  quadrants,  the  needle  will  tend  to 
be  deflected  by  the  attraction  and  repulsion  of  the  charges.  The 
force  with  which  the  needle  tends  to  turn  may  be  measured  by  a 
torsion  head  as  in  an  electrodynamometer,  or  by  suspending  the 
needle  so  that  a  certain  portion  of  its  weight  must  be  lifted  as  it 
turns.  If  the  two  poles  of  a  battery,  for  instance,  be  connected  to 
the  two  terminals  of  the  electrometer,  one  terminal  is  brought  to  a 
high  pressure  and  the  other  to  a  low  pressure  on  account  of  the  action 
of  the  battery,  and  they  therefore  hold  corresponding  positive  and 
negative  charges.  The  deflection  of  the  needle  indicates  the  pressure 
developed  by  the  battery.  This  pressure  may  be  directly  read  off  in 
volts  if  the  instrument  has  been  properly  calibrated.  In  the  same 
way  if  the  two  ends  of  a  resistance  through  which  a  current  is  flow- 
ing, such  as  an  electric  lamp,  be  connected  to  the  electrometer,  one 
terminal  is  brought  to  a  high  and  the  other  to  a  low  pressure  and  the 
deflection  of  the  needle  shows  the  difference  of  pressure  between  the 
ends  of  the  resistance.  Electrometers  made  for  use  in  everyday 
measurements  of  electric  pressure  are  usually  called  electrostatic 


82 


voltmeters  and  are  used  to  some  extent,  particularly  for  measuring 
alternating  electric  pressures.  They  can  be  used  for  the  latter  pur- 
pose, since  the  polarity  of  the  two  pairs  of  quadrants  and  oi  the 
needle  change  at  the  same  instant  and  consequently  the  needle  is 
deflected  continously  in  the  same  direction.  Fig.  62  shows  an  elec- 
trostatic voltmeter  made  for  measuring  pressures  of  several  thousand 
volts. 

Still  another  method  of  measuring  an  electric  pressure  is  to 
compare  it  with  a  standard  pressure.  If  between  the  points  whose 
difference  of  pressure  it  is  desired  to  measure,  a  known  large  resist- 
ance be  connected,  a  small  current  will  flow  through  the  resistance, 
and  the  pressure  will  fall  along  the  path  of  the  current  in  proportion 
to  the  resistance  passed  over.  Now  suppose  the  terminals  of  a  bat- 
tery cell  be  connected  in  series  with  a  galvanometer  to  certain  points 
on  the  resistance  (Fig.  63)  in  such  a  way  that  the  pressure  of  the  cell 
is  in  opposition  to  the  difference  of  pressure  between  the  poLits.  If 
the  latter  pressure  be  greater  than  that  of  the  cell,  a  current  will  flow 
through  the  cell  and  galvanometer,  and  the  galvanometer  needle 
will  be  deflected.  The  same  thing  will  occur  if  the  pressure  of  the 
cell  is  the  greater,  but  the  current  will  be  reversed.  Finally,  if  the 
portion  of  the  resistance  which  is  between  the  terminal  connections 
of  the  cell  be  so  adjusted  that  no  current  flows  through  the  galvano 
meter,  the  fall  of  pressure  through  that  part  of  the  resistance  exactly 
equals  xthe  pressure  produced  by  the  cell.  The  total  pressure  to  be 
measured  is  then  equal  to  the  pressure  developed  by  the  cell  multi- 
plied by  the  ratio  of  the  total  resistance  to  the  balancing  resistance. 
In  the  figure  the  pressure  of  the  cell  is  marked  1.2  volts,  the  total  re- 
sistance is  10,000  ohms  and  the  balancing  resistance  is  100  ohms. 
Assuming  a  balance,  the  total  pressure  must  be  1.2x10,000  100=120 
volts. 

A  special  arrangement  for  measuring  pressures  by  comparison  is 
often  called  a  potentiometer,  and  the  cells  used  for  the  comparison  are 
called  standard  cells.  It  is  evident  that  standard  cells  must  develop 
a  very  uniform  pressure  under  all  conditions  of  their  use.  The  best 
standard  cell  is  that  called  Clark* s  cell,  after  its  inventor.  This  was 
recommended  by  the  Chicago  Electrical  Congress  to  be  used  as  a 
comparative  standard  of  pressures,  and  its  pressure  was  given  in 
accordance  with  experimental  tests  to  be  1.434  volts  at  15°  Cen- 
tigrade when  set  up  according  to  fixed  instructions.  Professor  Car- 
hart  has  endeavored  to  make  a  standard  cell  with  exactly  one  volt 
pressure.  Voltmeters  have  been  made  upon  the  principle  of  a  poten- 
tiometer. 

Electric  currents  may  be  indirectly  measured  by  means  of  a  volt- 
meter, and  a  known  resistance  placed  in  the  circuit  through  which 
the  current  flows.  In  this  case  the  voltmeter  is  used  to  measure  the 
difference  of  pressure  between  the  ends  of  the  resistance,  and  the 
current  may  be  at  once  calculated  from  Ohm's  law. 

Copyrighted.  1894 

83 


The  National  School  of  Electricity. 

REVIEW  OF  LESSON  XL, 

Points  for  Review.    1.     What  are  the  three  effects  by  which  electric  currents  may 
directly  measured? 

2.  What  are  amperemeters? 

3.  What  is  a  milliampere?     What  is  a  microampere? 

4.  What  are  the  three  classes'of  magnetic  amperemeters? 

5.  What  is  the  general  arrangement  of  Weston  amperemeters? 

6.  What  are  electrodynamometers? 

7.  What  are  hot  wire  instruments? 

8.  How  are  the  scales  of  amperemeters  divided? 

9.  By  what  instruments  can  alternating  currents  be  measured?     Why? 

10.  How  may  very  large  currents  be  conveniently  measured? 

11.  What  is  the  commonest  method  of  measuring  electric  pressures? 

12.  What  are  voltmeters? 

13.  What  is  the  principle  of  the  Cardew  voltmeter? 

14.  How  may  electrometers  be  used  to  measure  electric  pressures? 

15.  What  are  electrostatic  voltmeters? 

16.  How  may  standard  cells  be  used  in  measuring  electric  pressures? 

17.  How  may  a  current  be  measured  by  using  a  voltmeter  and  a  standard  resistance? 


XII. 

EVERY-DAY  MEASUREMENTS  OF  ELECTRIC  POWER. 
CONDENSERS  AND  THE  MEASUREMENT  OF  THEIR 

CAPACITY. 

The  electric  power  which  is  used  in  any  part  of  a  circuit,  may 
be  determined  by  measuring  by  an  amperemeter  the  curient  flowing, 
and  by  a  voltmeter  the  pressure,  or  voltage  as  it  is  often  called,  at 
the  terminals  of  the  portion  of  the  circuit.  These  being  multiplied 
together,  give  the  power  in  watts  (Lesson  8,  page  52).  Instruments 
are  made  in  which  the  double  measurement  and  multiplication  is  all 
made  together,  so  that  their  indications  are  directly  proportional  to 
power.  Such  instruments  are  called  wattmeters  because  they  meas- 
ure watts.  The  simplest  form  of  wattmeter  is  an  electrodynamo- 
meter  in  which  one  coil  is  wound  with  many  turns  of  fine  wire 
exactly  as  though  it  were  to  be  used  as  a  voltmeter  coil,  and  the 
other  coil  is  wound  as  though  it  were  to  used  in  an  amperemeter. 
For  convenience  we  will  call  them  \h^  pressure  coi/and  the  current  coil. 
The  action  of  such  a  wattmeter  is  best  explained  by  an  illustration. 
Suppose  it  is  desired  to  measure  the  power  used  by  an  electric  motor, 
then  the  current  coil  of  the  wattmeter  is  connected  in  series  with 
the  motor,  and  the  pressure  coil  is  connected  across  the  termi- 

84- 


nals  of  the  motor.  The  magnetic  ef- 
fect of  the  current  coil  is  therefore  pro- 
portional to  the  current  which  flows 
through  the  motor,  and  that  of  the 
pressure  coil  is  proportional  to  the 
pressure  at  which  the  current  is  sup- 
plied to  the  motor.  The  indications  of 
an  electrodynamometer  are  propor- 
tional to  the  product  of  the  magnetic 
effects  of  the  two  coils  (Lesson  n,  page 
76).  Consequently,  in  this  case  the  in- 
dications are  proportional  to  current 
times  pressure  or  watts,  instead  of  cur- 
rent times  current,  as  in  the  Siemens 
electrodynamometer  (Lesson  u,  page  FIG.  t>4. 


Wattmeters  may  be  calibrated  by  comparing  their  readings, 
when  connected  to  a  circuit,  with  the  indications  of  standard  volt- 
meters and  amperemeters.  By  proper  construction  and  adjustment 
of  their  scales  they  may  be  made  direct  reading. 

It  is  also  possible  to  make  electrostatic  wattmeters,  and  watt- 
meters based  upon  other  principles. 

Recording  wattmeters  may  be  used  to  show  the  amount  of  power 
used  each  month  by  the  customers  of  electric  plants.  The  com- 
monest form  of  wattmeter  used  for  this  purpose  is  that  shown  in 
Fig.  64,  known  as  the  Thomson  wattmeter,  after  its  inventor.  This 
consists  of  a  little  electric  motor  without  any  iron  in  its  workin'g 
parts,  which  is  arranged  with  its  revolving  part  or  armature  as  a 
pressure  coil,  and  its  magnetizing  coil  as  a  current  coil.  The  mag- 
netic pull  which  tends  to  make  the  armature  rotate  is  proportional 
to  the  product  of  the  two  magnetizing  effects,  and  this  is  proportional 
to  the  watts  in  the  circuit,  exactly  as  in  an  electrodynamometer.  If  the 
speed  of  such  an  armature  is  made  to  be  proportional  to  the  magnetic 
pull  it  is  easily  seen  that  every  revolution  of  the  armature  means  a  cer- 
tain number  of  watts  used  for  a  fixed  length  of  time.  Such  instruments 
usually  have  attached  to  the  spindle  of  the  armature  a  set  of  dials 
like  those  of  a  gas  meter  which  record  the  revolutions  and  are  so 
marked  that  the  consumption  of  electric  power  may  be  recorded  in 
watt- hours.  Watt-hours  are  the  product  of  the  number  of  watts  by 
the  number  of  hours  during  which  the  power  is  used.  If  no  txter- 
nal  retarding  force  were  applied  to  the  armature  of  such  an  instru- 
ment it  would  run  away  as  soon  as  placed  in  service,  and  in  order  that 
its  speed  may  be  proportional  to  the  watts  the  retarding  force  must 
be  proportional  to  the  speed.  This  Is  very  ingeniously  arranged  in 
the  Thomson  recording  wattmeter  by  placing  at  the  bottom  of  the 
spindle  a  flat  disc  of  copper  on  either  side  of  which  are  placed  the 
poles  of  magnets.  The  rotation  of  the  disc  between  the  magnet 

86 


poles  generates  electric  currents  in  it  which  are  attracted  by  the 
magnets  and  retard  the  motion  of  the  disc. 

Other  meters  for  use  in  determining  the  amount  of  power  con- 
sumed by  customers,  which  are  externally  similar  to  the  Thomson, 
only  read  ampere-hours.  An  ampere  hour  is  equal  to  3,600  ampere- 
seconds,  but  one  ampere-second,  or  one  ampere  flowing  for  one 
second,  means  the  transfer  of  one  coulomb  of  electricity  through  the 
circuit.  Consequently  the  readings  of  meters  which  record  in  ampere- 
hours  are  directly  comparable  with  the  indications  of  the  Edison  elec- 
trolytic meter  which  has  been  mentioned  before  (L,esson  9,  page  64). 
Meters  which  read  in  ampere-hours  are  sometimes  called  coulomb- 
meters.  The  reading  of  ampere  hours  has  no  relation  to  the  power  con- 
sumed in  a  circuit  unless  the  pressure  in  a  circuit  is  known,  but  in  the 
cases,  where  such  meters  are  used  the  pressure  is  intended  to  be  kept 
at  a  constant  known  value  so  that  the  watt-hours  used  by  each  customer 
may  be  easily  determined,  when  desired,  by  multiplying  the  ampere- 
hour  reading  of  his  meter  by  the  pressure  in  the  circuit. 

If  the  bottom  of  a  cylindrical  can,  filled  with  water,  be  connected 
by  means  of  a  tube  to  the  bottom  of  a  similar  can  of  different  diame- 
ter standing  on  the  same  level,  the  water  will  flow  into  the  second 
can  until  it  stands  at  the  same  height  in  both.  The  quantity  of 
water  in  each  vessel  when  the  flow  has  ceased  is  proportional  to  the 
capacity  of  the  vessel*  During  the  flow  the  water  falls  in  one  can 
and  rises  in  the  other.  In  the  same  way  if  a  conductor,  such  as  a 
brass  ball  carrying  an  electric  charge  be  touched  by  an  uncharged  con- 
ductor, part  of  the  charge  flows  to  the  second  conductor.  During  the 
flow  the  electric  pressure  of  one  conductor  falls  and  the  pressure  of  the 
other  rises.  After  the  flow  has  ceased  the  electrical  pressure  of  the  two 
conductors  is  equal  (compare  Lesson  2,  page  13).  The  quantity  of 
electricity  on  the  two  conductors  is  not  equal  unless  the  conductors 
are  exactly  similar,  but  the  quantity  on  each  will  depend  upon  its 
capacity  to  hold  electricity,  or  its  electrical  capacity.  The  electrical 
capacity  of  a  conductor  depends  upon  its  size,  shape,  and  surround- 
ings. It  is  measured  by  the  number  of  coulombs  of  electricity  required 
to  raise  the  electrical  pressure  of  the  conductor  one  volt,  exactly  as  the 
capacity  of  a  cylindrical  can  is  measured  by  the  number  of  gallons  of 
water  required  to  fill  it  to  the  depth,  or  head,  of  one  foot. 

When  the  pressure  of  a  conductor  is  raised  one  volt  by  the 
charge  of  one  coulomb,  the  conductor  is  said  to  have  a  capacity  of 
one  farad,  after  Faraday,  the  distinguished  English  scientist. 

The  electrical  pressure  of  a  conductor  carrying  a  charge  of  elec- 
tricity is  ordinarily  reckoned  as  the  difference  between  it  and  the 
average  electrical  pressure  of  the  earth's  surface,  which  is  called  zero. 
This  is  similar  to  the  reference  of  levels  or  heights  to  the  sea  level  as 
a  zero  point  from  which  to  start.  The  electrical  pressure  of  a  charged 
conductor  cannot  be  measured  by  an  ordinary  voltmeter  since  the 


charge  would  be  at  once  dissipated  by  the  current  which  would  flow 
through  the  voltmeter  when  connected  between  the  conductor  and  the 
earth.  The  pressure  may,  however,  be  measured  by  a  sufficiently  sen- 
sitive electrometer  or  electrostatic  voltmeter.  For  instance,  in  the  case 
of  a  quadrant  electrometer  which  was  briefly  described  in  the  preceding 
lesson  (Lesson  n,  page  82),  the  needle  and  its  pair  of  quadrants  may 
be  connected  to  earth  and  the  other  pair  of  quadrants  to  the  charged 
body.  Then  if  the  instrument  is  sufficiently  sensitive  the  needle  will 
be  deflected  an  amount  which  is  proportional  to  the  difference 
between  the  earth's  electrical  pressure  and  that  of  the  charged  body. 

The  presence  of  charges  of  an  opposite  sign  near  a  charged  con- 
ductor has  a  remarkable  influence  on  the  conductor's  capacity.  For 
instance,  if  pieces  of  tin-foil  are  pasted  upon  the  two  sides  of  a  sheet 
of  mica  and  the  two  tin-foil  coatings  are  given  opposite  charges,  the 
charges  act  inductively  on  each  other  and  consequently  increase  their 
capacities.  Such  an  arrangement  is  called  a  condenser.  The  tin-foil 
sheets  are  called  the  coatings  or  plates  of  the  condenser  and  the  insu- 
lating material  is  called  the  dielectric.  The  coatings  of  a  condenser 
may  be  made  of  any  conducting  material,  and  the  dielectric  of  any 
insulating  material. 

The  combined  capacity  of  the  coatings  is  the  capacity  of  the  con- 
denser. A  condenser  has  a  capacity  of  one  farad  when  a  charge  of 
one  coulomb  of  electricity  raises  the  difference  of  electrical  pressure, 
or  potential,  of  the  plates  by  one  volt.  To  charge  a  condenser  with 
a  certain  quantity  of  electricity  means  that  a  positive  charge  of  the 
given  quantity  is  placed  upon  one  plate  and  an  equal  negative  charge 
on  the  other. 

A  condenser  may  be  charged  in  either  of  two  ways:  ist,  by  con- 
necting one  plate  to  earth  and  placing  the  charge  on  the  other  plate, 
when  the  required  opposite  charge  will  collect  on  the  grounded  plate 
by  induction;  and,  by  connecting  the  two  plates  of  the  condenser  to 
the  two  terminals  of  an  electric  battery,  or  other  source  of  electricity, 
when  the  charge  is  communicated  by  the  action  of  the  battery. 

Every  electrical  conductor,  as  we  have  seen,  has  capacity,  and 
when  an  insulated  wire  is  laid  in  the  earth  or  is  strung  over- 
head it  becomes  one  plate  of  a  condenser.  The  other  plate  of  the 
condenser  is  the  earth,  and  the  dielectric  is  the  insulating  covering 
of  the  wire,  or  the  air  which  is  between  it  and  the  earth.  The  capa- 
city of  a  wire  has  a  great  deal  of  effect  on  its  usefulness  in  telephone 
service.  Every  hundredth  of  a  microfarad  per  mile  of  conductor 
reduces  very  considerably  the  distance  through  which  the  telephone 
will  work  satisfactorily.  The  capacity  of  ocean  cables  is  also  a  matter 
of  much  importance,  and  capacity  effects  are  of  importance  in  teleg- 
raphy and  in  the  transmission  of  power  by  alternating  currents  of 
electricity. 

The  capacity  of  a  condenser  depends  directly  upon  the  area  of 


87 


its  plates,  their  closeness  together,  and  the  specific  inductive  capacity 
of  the  dielectric.  Different  insulating  materials  have  very  different 
values  as  dielectrics.  The  inductive  action  seems  to  be  stronger 
through  some  materials  than  through  others,  and  it  is  less  active 
through  air  than  through  any  solids  or  liquids.  Consequently  a 
condenser  which  has  air  for  a  dielectric  has  less  capacity  than  one  of 
exactly  equal  size  with  a  solid  dielectric.  The  ratio  of  the  capacities  of 
two  such  condensers  is  called  the  specific  inductive  capacity  of  the  solid 
dielectric.  The  annexed  table  gives  the  approximate  specific  induc- 
tive capacities  of  various  materials.  That  of  air  is  taken  as  unity  as 
a  matter  of  reference,  because  the  inductive  effect  is  less  through  it 
than  through  any  common  substance. 

SPECIFIC  INDUCTIVE  SPECIFIC  INDUCTIVE 

MATERIAL,.  CAPACITY.  MATERIAL.  CAPACITY. 

Air i.  Gutta-percha 2.5 

Petroleum 2.1  Shellac 2.9 

Turpentine 2.2  Sulphur  ...  3.7 

Rubber 2.3  Mica -,. 6.6 

Paraffine 2.3  Glass 5.0  to  10.0 

The  table  shows  the  importance  of  carefully  selecting  the  insu- 
lation for  telephone  cables  in  order  that  their  capacities  may  be  the 
least  possible.  In  fact,  the  insulation  directly  surrounding  the  indi- 
vidual wires  of  such  cables  is  often  made  from  crinkled  paper  so 
that  air  makes  up  a  considerable  part  of  the  material  between  the 
wires.  While  glass  is  one  of  the  best  of  insulators,  it  is  one  of  the 
poorest  materials  to  use  for  the  continuous  insulation  of  the  wires  in 
telephone  cables  on  account  of  its  great  specific  inductive  capacity. 

Insulated  wires  and  cables  placed  underground  always  have  a 
much  greater  capacity  than  wires  of  the  same  size  and  length  placed 
overhead.  This  is  largely  because  the  dielectric  of  the  underground 
wires  is  so  much  thinner  than  that  of  the  overhead  wires,  and  par- 
tially because  the  inductive  capacity  of  solid  dielectrics  is  greater 
than  that  of  air.  The  capacity  of  an  overhead  wire  strung  at  a 
height  of  thirty  feet  above  the  ground  is  only  about  one  twentieth  of 
that  of  a  similar  wire  well  insulated  with  a  rubber  compound  and 
placed  underground,  and  only  about  one  tenth  of  that  of  a  similar 
wire  insulated  with  cotton  and  paraffine  and  placed  in  a  cable  under- 
ground. 

It  is  very  important  to  make  measurements  of  the  capacity  of 
conductors  to  be  used  in  telephony  and  telegraphy.  This  may  be 
done  in  various  ways,  but  the  method  that  is  generally  used  is  to 
directly  compare  the  capacity  of  the  wire  with  that  of  a  standard 
condenser  by  means  of  a  ballistic  galvanometer.  Standard  condens- 
ers are  made  of  various  capacities  and  put  up  in  boxes  so  that  they 
may  be  readily  used  for  various  purposes.  Since  a  capacity  as  large 
as  a  farad  is  very  seldom  met  in  the  electrical  industries,  standard 


88 


FIG.  65. 

condensers  are  usually  made  equal  to  microfarads  (one  millionth  of  a 
farad)  or  fraction  of  microfarads,  and  the  microfarad  has  become  the 
common  unit  in  which  capacities  are  measured.  Fig.  65  shows  an  ad- 
justable condenser  which  is  made  with  five  divisions  of  .  i  microfarad 
each.  The  five  divisions  may  be  put  in  parallel  so  that  the  total 
capacity  is  y2  microfarad.  Since  the  capacity  of  a  condenser  is  directly 
proportional  to  the  area  of  the  plates,  connecting  condensers  in  parallel 
gives  a  total  or  combined  capacity  which  is  equal  to  the  sum  of  the  in- 
dividual capacities.  Again,  since  the  capacity  depends  inversely 
upon  the  thickness  of  the  dielectric,  connecting  condensers  of  equal 
capacity  in  series,  gives  a  combined  capacity  equal  to  the  capacity  oj 
one  condenser  divided  by  the  number  in  series,  because  connecting 
condensers  in  series  has  the  effect  of  adding  together  the  thickness 
of  the  dielectrics  in  the  different  condensers.  Where  condensers  of 
different  capacities  are  connected  together  in  series,  the  combined 
capacity  is  equal  to  the  reciprocal  of  the  sum  of  the  reciprocals  of  the 
individual  capacities.  ( * ,  K  = J ; k  x  -j- x /  k  2  + 1 ,  k  3 . )  (Compare  combi ned 
resistances,  Lesson  7,  page  46).  Condensers  connected  in  series  are 
sometimes  said  to  be  connected  in  cascade. 

The  plates  of  standard  condensers  are  usually  made  of  .tinfoil, 
and  the  dielectric  of  mica,  paraffined  paper,  or  oiled  paper. 

A  ballistic  galvanometer  is  simply  a  sensitive  galvanometer 
which  is  not  dead  beat.  In  this  case,  if  a  certain  quantity  of  elec- 
tricity be  passed  through  the  coils  of  the  instrument  in  a  very  short 
interval  of  time,  its  magnetic  effect  on  the  needle  is  very  much  like 
that  of  a  blow,  while  the  magnetic  effect  of  a  steady  current  on  the 


needle  is  like  that  of  a  steady  push.  The  needle  of  a  galvanometer 
where  such  a  transient  current  or  discharge  passes  through  it,  swings 
off  through  an  angle  which  is  proportional  to  the  quantity  of  elec- 
tricity in  the! discharge,  provided  the  angle  of  swing  or  throw  is  not 
too  great.  To  measure  the  capacity  of  a  cable,  a  standard  condenser 
is  selected  of  a  capacity  nearly  equal  to  that  of  the  cable.  The  con- 
denser is  charged  by  a  few  cells  of  battery,  and  by  means  of  a  key 
its  connections  are  then  changed  so  that  it  discharges  through  a 
galvanometer.  The  throw  of  the  galvanometer  needle  is  observed. 
The  same  battery  is  now  connected  with  one  terminal  to  the  cable 
conductor  and  its  other  terminal  to  the  cable  sheathing  or  to  the 
earth.  In  this  way  the  cable  is  charged.  The  cable  and  earth  con- 
nections are  then  transferred  to  the  galvanometer  by  means  of  the 
key,  and  the  cable  is  discharged  through  the  galvanometer.  The 
throw  of  the  needle  is  again  observed.  The  two  throws  are  propor- 
tional to  the  quantities  of  electricity  in  the  charges  of  the  condenser 
and  the  cable.  Since  these  were  charged  by  the  same  battery  and 
therefore  to  the  same  pressure,  the  quantities  of  electricities  are  pro- 
portional to  the  respective  capacities.  Therefore  the  capacities  are 
proportional  to  the  throws. 

The  object  of  taking  a  condenser  of  a  capacity  nearly  equal  to 
that  of  the  cable  is  to  make  the  throws  nearly  alike  and  thus  avoid 
instrumental  errors.  When  a  proper  condenser  cannot  be  obtained, 
a  shunt  may  be  used,  but  this  is  also  likely  to  introduce  errors  when 
used  with  discharges.  The  insulation  of  the  instruments  and  their 
connections  must  be  as  perfect  as  possible  in  capacity  tests,  as  is  also 
necessary  in  insulation  tests  (Lesson  10,  page  72). 

As  an  example,  suppose  the  discharge  of  a  y2  microfarad  con- 
denser when  charged  by  five  cells  gives  a  galvanometer  throw  of  200 
divisions;  and  when  a  cable  two  miles  long  is  charged  by  the  same 
cellsj  arM  discharged  through  the  galvanometer,  the  throw  is  180. 
Then  the  capacity  of  the  cable  is  iff  X^=-45  microfarads,  and  the 
capacity  of  the  cable  per  mile  is  .45  /  2=.  225  microfarads. 

A  Ley  den  jar  (Fig.  66)  is  a  condenser  made  out.  of  a  glass  jar 
which  is  coated  with  tin  foil  both  outside  and  inside. 

Copyrighted,  1894, 


90 


The  National  School  of  Electricity. 

REVIEW   OF   LESSON    XII. 

Points  for  Review.     1.     What  are  wattmeters? 

2.  How  may  an  electrodynamometer  be  used  as  a  wattmeter? 

3.  For  what  purpose  are  recording  wattmeters  and  coulomb-meters  used? 

4.  What  is  a  watt-hour?     An  ampere-hour? 

5.  What  is  electrical  capacity? 

6.  What  is  a  farad?     Why  is  a  microfarad  commonly  used  as  the  unit  for  measur- 
ing capacity? 

7.  What  is  a  condenser?     What  is  the  capacity  of  a  condenser? 

8.  How  may  a  condenser  be  charged? 

9.  Upon  what  does  the  capacity  of  a  condenser  depend? 

10.  What  is  the  specific  inductive  capacity  of  a  substance? 

11.  Why  must  the  insulating  material  for  telephone  cables  be  carefully  selected  to 
avoid  excessive  capacity? 

12.  Why  is  glass  a  poor  material  to  use  for  continuously  insulating  telephone  cables? 

13.  Why  do  underground  wires  have  a  greater  capacity  than  similar  wires  strung 
overhead? 

14.  If  three  condensers  of  ^  microfarad  be  connected  in  parallel,  what  is  their  com- 
bined capacity? 

15.  How  may  capacities  be  compared  by  using  a  ballistic  galvanometer? 

16.  Suppose  that  a   l/$    microfarad  condenser  causes  a  throw  of  80  divisions  when 
charged  with  three  cells  and  discharged  through  a  galvanometer,  while  the  throw  caused 
by  a  five-mile  wire,  when  charged  by  the  same  battery,  is  120  divisions;  what  is  the 
capacity  of  the  wire  per  mile? 

17.  What  is  a  leyden  jar? 


LESSON    XIII. 

ELECTROLYTIC  DEPOSITION  OF  METALS. 

The  electrochemical  operations  which  result  in  depositing 
metals  from  a  solution  of  their  metallic  salts  are  very  wide-spread 
in  the  industries  and  are  of  great  usefulness.  The  magnitude  of  the 
works  involved  in  most  of  the  operations  does  not  approach  that  of 
works  built  for  the  purpose  of  furnishing  electricity  for  light  and 
power;  nor  do  the  ordinary  electrolytic  operations  appeal  to  the  ordi- 
nary observer  as  do  the  applications  of  electricity  to  transmitting 
messages,  driving  street  cars,  or  furnishing  light  or  power.  Never- 
theless we  owe  to  electrochemical  operations  many  of  the  common, 
est  necessities  and  comforts  of  life.  The  commercial  applications  of 
electrolysis  cover  a  wide  and  useful  range  from  nickel  and  silver 
Elating  to  electrotyping  for  the  use  of  the  printer;  and  from  methods 
of  bronzing  and  gilding  to  methods  of  smelting  certain  ores  and 


91 


refining  metals.  Lesson  13  will  be  given  to  the  consideration 
of  the  commonest  and  most  useful  of  these  applications.  Nearly 
all  of  the  processes  depend  upon  the  laws  of  chemical  action 
which  have  already  been  described  in  the  lessons  on  electric 
batteries  and  voltameters  (Lessons  3,  4  and  12),  but  frequently 
the  solutions  used  are  quite  complex,  so  that  the  chemical  action 
which  occurs  is  complicated  and  not  always  fully  understood. 

A  working  knowledge  of  the  processes  of  electrodeposition  of 
metals  has  been  possessed  only  since  1800,  and,  indeed,  many  of  the 
more  important  processes  of  plating,  electrotyping,  etc.,  have  been 
discovered  since  1840  or  1845,  while  some  of  the  important  opera- 
tions of  electrometallurgy,  such  as  the  electrolytic  recovery  of 
aluminum  'and  the  commercial  refining  of  copper  by  electrolysis, 
have  not  been  employed  until  within  a  very  few  years.  The  next 
few  years  seem  destined  to  see  electrolysis  and  electrometallurgical 
processes  (processes  of  treating  metals  in  which  electricity  is  used) 
put  into  extended  use  in  the  recovery  of  various  metals  from  their 
ores,  and  in  some  hitherto  little  explored  fields,  such  as  the  purifying 
of  drinking  water  and  sterilising  of  sewage. 

Electroplating  is  the  process  of  covering  articles  of  metal  with  a 
thin  layer  of  another  metal  by  means  of  electrolysis.  The  covering 
usually  consists  of  nickel,  silver,  or  gold,  and  the  base  or  covered 
metal  is  ordinarily  of  some  composition  such  as  white  metal,  Britan- 
nia metal,  german  silver,  or  brass.  The  details  of  the  process  are 
quite  different  for  the  different  metals  used  in  plating.  We  will  first 
take  up  silver  plating,  as  silver  is  the  most  important  metal  in  plat- 
ing processes.  The  commonest  salts  of  silver  are  chloride  of  silver, 
nitrate  of  silver,  cyanide  of  silver,  acetate  of  silver,  sulphide  of  silver, 
and  oxide  of  silver.  A  salt  of  a  metal  is  a  chemical  combination 
formed  by  the  action  of  an  acid  on  the  metal.  Thus,  sulphide  of 
silver  is  a  combination  of  sulphur  and  silver,  and  nitrate  of  silver  is 
formed  by  the  chemical  action  of  nitric  acid  upon  silver.  Nitric  acid 
is  a  chemical  combination  of  hydrogen  with  oxygen  and  nitrogen, 
the  oxygen  and  nitrogen  in  this  case  forming  what  is  called  an  acid 
radical.  The  radical  of  nitric  acid  has  a  greater  chemical  attraction 
or  affinity  for  silver  than  for  hydrogen.  Consequently  when  silver  is 
immersed  in  nitric  acid  the  silver  is  attacked  and  dissolved,  during 
which  process  it  combines  with  the  acid  radical  and  forms  nitrate  of 
silver,  while  the  hydrogen  of  the  acid  is  given  off.  The  salts  of  silver 
which  are  used  in  electroplating  are  usually  made  from  the  nitrate. 
The  nitrate  of  silver  is  produced  by  adding  pure  silver  in  small  quan- 
tities at  a  time,  to  a  warm  mixture  of  one  measure  of  distilled  water 
to  four  measures  of  the  strongest  pure  nitric  acid.  The  action  of  the 
acid  upon  the  silver  is  very  intense  and  causes  much  heat  to  be  given 
off  (compare  the  action  of  sulphuric  acid  upon  zinc,  Lesson  3,  page  15) 
and  if  the  mixture  be  too  hot  or  too  much  silver  be  added,  the  liquid 


92 


may  boil  over.  In  this  case  the  mixture  may  be  cooled  by  adding  a 
little  cold  distilled  water.  When  the  mixture  will  dissolve  no  more 
silver  the  solution  may  be  put  in  a  covered  jar  and  set  in  a  dark  place 
until  it  is  required  for  use. 

For  use  with  a  silver  voltameter  a  properly  diluted  solution  of 
nitrate  of  silver  is  used  (Lesson  9,  page  64),  but  the  deposit  from  a 
nitrate  solution  does  not  make  a  satisfactory  plating.  The  best  silver 
plating  solution  is  one  containing  cyanide  of  silver.  Cyanide  of  silver 
is  the  salt  formed  by  the  combination  of  silver  with  prussic  acid. 
A  solution  of  cyanide  of  silver  is  formed  by  slowly  adding  to 
the  silver  nitrate  solution  made  substantially  as  already  described 
a  weak  solution  of  cyanide  of  potash  or  white  prussiate  of 
potash.  The  cyanide  of  potash  used  should  be  dissolved  in  about  ten 
times  its  own  weight  of  distilled  water.  The  addition  of  the  potash 
solution  to  the  nitrate  of  silver  solution  should  be  continued  as  long 
as  a  white  precipitate  forms,  but  no  longer,  or  some  of  the  silver  is 
lost.  The  precipitate  which  forms  is  cyanide  of  silver.  This  should 
be  allowed  to  settle,  after  which  the  clear  liquid  may  be  carefully 
poured  or  drawn  off.  The  precipitate  is  then  washed  a  number  of 
times  by  pouring  distilled  water  over  it  and  stirring,  allowing  the 
precipitate  to  settle  and  pouring  off  the  liquid.  Cyanide  of  silver 
does  not  dissolve  in  water  but  readily  dissolves  in  a  solution  of  cyanide 
of  potash  in  water,  aud  silver  plating  solutions  are  usually  made  by 
so  dissolving  the  silver  cyanide.  Cyanide  solutions  are  extremely 
poisonous  and  therefore  must  be  handled  carefully,  and  on  account  of 
the  value  of  the  silver  which  they  contain  must  be  handled  without 
waste. 

The  vats  in  which  silver  plating  operations  are  carried  out  are 
usually  made  of  wood,  though  they  are  sometimes  made  of  sheet-iron 
lined  with  wood.  They  are  of  various  dimensions,  but  generally  are 
from  two  to  three  feet  wide,  five  to  six  feet  long,  and  about  thirty 
inches  deep.  When  the  solution  is  made  up  and  put  in  the  vat  for 
service  it  usually  does  not  require  changing  for  a  number  of  years. 
It  sometimes  requires  filtering,  and  the  addition  of  water  to  supply 
that  lost  by  evaporation,  or  the  addition  of  cyanide  salts  to  supply 
losses  which  have  come  about  by  electrolysis.  The  exact  proportions 
of  the  solutions  used  for  silver  plating  in  different  factories  vary  con- 
siderably, but  they  are  nearly  always  substantially  as  already  described. 
The  general  arrangement  of  a  plating  vat  is  shown  in  Fig.  67, 
where  the  flat  plates  inside  the  vat  are  sheets  of  silver  which  are  con- 
nected to  the  positive  pole  of  the  source  of  current,  and  form  the 
anodes  of  the  electrolytic  cell  (Lesson  9,  page  64),  whilethe  spoons, 
forks,  and  the  other  articles  to  be  plated  form  the  cathode.  The 
supports  for  the  anodes  and  cathodes  are  usually  made  of  brass  or 
copper  tubes  laid  across  the  top  of  the  vat.  The  articles  to  be  plated 
are  ordinarily  supported  on  looped  pieces  of  insulated  copper  wire 


93 


FIG.  67. 


FIG.  68. 

(Fig.  68).  The  insulation  of  these  supports  where  they  are  immersed 
in  the  liquid  is  important  in  order  to  avoid  an  unnecessary  and  ex- 
pensive deposit  of  silver  upon  them.  The  silver  deposit  made  on  the 
cathodes  occurs  as  a  result  of  electrolysis,  and  an  equal  amount  of 
silver  goes  into  the  liquid  from  the  anode  when  all  is  working  well 
(compare  Lesson  9,  page  64). 

The  quality  of  the  deposit  which  is  made  in  electroplating  is  of 
the  first  importance.  The  three  points  to  be  looked  after  most  care- 
fully are  the  strength  of  the  current  as  compared  with  the  magni- 
tude of  the  surface  to  be  plated,  the  composition,  density,  and  tem- 
perature of  the  plating  solution,  and  the  condition  of  the  articles  to 
be  plated  when  put  into  the  solution.  The  current  for  plating  was 
formerly  furnished  by  batteries  but  it  is  now  ordinarily  furnished 
from  small  dynamos  which  produce  a  low  pressure  properly 
adapted  for  its  purpose.  The  pressure  may  also  be  adjusted  to  a  con- 
siderable extent  by  means  of  a  resistance  box  connected  in  circuit 


with  the  magnetizing  coils  oi  the  dynamo.  The  current  and  pres- 
sure supplied  by  the  dynamo  may  be  measured  by  means  of  an 
amperemeter  and  a  voltmeter.  One  dynamo  of  sufficient  size  maybe 
used  to  supply  current  to  several  plating  vats.  The  vats  may  be 
connected  either  in  series  or  in  parallel,  depending  upon  the  pressure 
developed  by  the  dynamo.  When  the  current  is  of  the  proper  amount 
the  covering  which  is  deposited  upon  the  plated  articles  is  hard,  white, 
adheres  closely,  and  is  deposited  with  reasonable  rapidity.  When 
the  current  is  too  small  the  deposit  usually  is  of  good  quality  but  the 
plating  progresses  too  slowly.  When  the  current  is  too  great  the 
plating  is  likely  to  become  gray  or  black  and  rough,  while  gas  is 
sometimes  given  off  at  the  cathode.  A  discoloration  of  the  silver 
deposit  may  also  occur  from  impurities  in  the  liquid.  Such  discolor- 
ation may  often  be  removed  by  proper  after  treatment  of  the  plated 
articles,  but  to  this  attention  cannot  be  given  here. 

The  form  of  the  articles  to  be  plated  often  has  much  to  do  with 
r.ie  quality  of  the  plating.  Thus  bulky  articles  with  a  given  sur- 
face often  do  not  plate  as  rapidly  as  thinner  articles  with  exactly  the 
same  amount  of  surface  to  be  covered.  Edges  and  points  often 
gather  a  granular  or  rough  deposit  while  the  flat  parts  of  the  same 
articles  take  a  satisfactory,  hard  deposit.  Such  difficulties  can  be 
overcome  only  by  making  a  proper  mutual  adjustment  of  the  dis- 
tances between  anodes  and  cathodes,  the  quality  of  the  liquid,  and 
the  current  per  unit  surface  of  the  articles.  When  articles  which  have 
great  irregularities  of  surface  are  to  be  plated,  the  distance  between 
anodes  and  cathodes  must  be  greater  than  that  which  is  satisfactory 
when  the  articles  have  a  uniform  surface,  otherwise  the  more  promi- 
nent points  of  the  articles  will  receive  a  heavy  deposit  while  the 
hollows  may  receive  little  or  no  deposit.  It  is  important  that  all 
plated  articles  be  given  a  uniform  deposit  of  proper  thickness  upon 
the  surfaces  which  it  is  desired  to  cover.  The  thickness  of  silver  plat- 
ing ordinarily  varies  from  the  thinnest  possible  coating  to  the  thickness 
of  thin  writing  paper,  depending  upon  the  quality  of  the  product. 

There  is  a  method  of  plating  by  simply  dipping  the  articles 
in  a  proper  silver  solution  which  is  used  to  silver  small  articles, 
such  as  hooks  and  eyes,  on  which  the  coating  is  too  thin  to  be 
really  measured.  In  this  case  the  plating  is  not  due  to  electro- 
lytic action,  but  simply  to  chemical  action  between  the  silver  solu- 
tion and  the  metal  composing  the  articles  to  be  covered.  This  is 
called  plating  by  simple  immersion. 

In  preparing  articles  for  silver  plating,  the  greatest  care  must  be 
taken  to  make  them  absolutely  clean  and  bright,  or  the  plating  will 
not  take  a  permanent  hold,  but  will  peel  off.  It  is  first  necessary  to 
prepare  the  articles  for  the  kind  of  coating  they  are  intended  to 
receive;  if  the  plating  is  intended  to  be  polished,  the  articles  must 
be  polished,  all  deep  scratches  must  be  removed,  etc.  This  may  be 


^<£e£* 
f  * 

iXTNI' 

K^ 


done  by  filing,  scouring,  polishing,  etc.  After  this  preparation  the 
cleaning  is  begun  by  dipping  in  a  warm  solution  of  caustic  potash 
or  soda  which  cleans  off  all  grease.  This  solution  is  made  by  dis- 
olving  commercial  lye  in  water,  and  it  may  be  used  continuously 
until  its  caustic  properties  are  used  up.  After  dipping  in  lye  the 
articles  are  washed  in.  water  and  are  then  sometimes  dipped  in  dilute 
acid  to  give  them  a  proper  surface.  They  are  next  washed  with 
great  care  and  then  placed  in  the  depositing  vat.  It  is  quite  common 
to  cover  articles  to  be  silver  plated  with  a  very  thin  coating  of  mer- 
cury. The  object  of  this  is  to  avoid  oxidation  of  the  articles  which 
causes  the  plating  to  peel.  Coating  with  mercury  is  called  quicking 
and  it  may  be  effected  by  dipping  the  articles  into  a  dilute  solution 
<*f  nitrate  of  mercury,  or  the  solution  of  some  other  mercury  salt. 


FIG.  69. 

During  the  operations  of  dipping,  the  articles  should  be  sup- 
ported upon  wires  or  in  wire  baskets.  They  should  not  be  touched 
with  the  fingers  since  the  points  so  touched  are  made  greasy  and  the 
deposit  will  not  take. 

After  the  plating  is  completed  in  the  bath  the  articles  must  be 
put  through  a  series  of  operations  to  give  the  plated  surface  the 
proper  finish.  This  is  largely  done  by  polishing  on  rapidly  revolv- 
ing" wheels  made  of  brass  wires,  leather,  and  canvas.  The  processes 
are  called  scratching,  buffing,  and  polishing.  The  same  tools  are 
used  for  polishing  the  articles  before  plating.  In  the  case  of  some 
articles  the  polishing  is  done  by  means  of  hand  burnishers,  which  are 
smooth  tools  made  of  steel,  agate,  or  similar  hard  materials.  Some 
rorms  of  burnishers  are  shown  in  Fig.  69. 

Plating  with  gold  is  carried  on  in  very  much  the  same  way  as 
plating  with  silver.  The  commonest  solution  is  of  cyanide  of  gold 
made  up  in  a  manner  quite  similar  to  that  used  in  making  up  the 
cyanide  of  silver  solution.  The  solution  is  generally  used  when  hot 
and  great  care  to  have  all  details  exactly  right  is  necessary  to  get  a 
deposit  of  satisfactory  color.  It  is  particularly  important  that  all  the 
materials  used  in  making  the  solution  shall  be  pure.  Gilding  the 
inside  of  silver  cups,  sugar  bowls,  and  cream  pitchers  is  commonly 
done  by  filling  the  article  to  be  gilded  with  the  hot  solution,  hang- 


96 


FIG.  70. 


FIG.  71. 


FIG.  73. 


FIG.  72. 


FIG.  74. 


ing  a  gold  anode  in  the  shape  of  a  cylinder  in  the  center  of  the  solu- 
tion, and  finally  connecting  up  a  battery  so  that  the  article  to  be 
gilded  is  the  cathode  (Fig.  70).  The  extreme  cleaning  of  articles  for 
gold  plating  is  usually  not  as  important  as  in  silver  plating,  since 
the  hot  solution  helps  in  the  cleaning. 

Nickel  plating  is  probably  the  most  generally  used  of  all  the 
different  styles  of  plating.  The  base  upon  which  nickel  is  plated  is 
usually  brass,  copper,  iron,  or  steel.  The  soft  white  metals  which 
are  often  silver  plated  are  seldom  nickel  plated.  The  hardness  of 
nickel  and  its  durable  polish,  give  to  nickel  plating  great  advantages 
for  use  in  the  finish  of  sanitary  appliances,  car  fittings  and  decora- 
tions, small  nuts,  bolts,  screws,  chains,  etc.,  used  in  small  machin- 
ery, bicycles,  stove  fronts,  metal  lamps,  and  many  similar  appliances. 
The  solution  which  is  used  in  nickel  plating  is  made  from  a  com- 
bined sulphate  of  nickel  and  sulphate  of  ammonia.  In  a  dry  state, 
this  is  ordinarily  known  as  nickel  salts  or  the  double  sulphate  of  nickel 
and  ammonia.  This  double  salt  may  be  purchased  in  the  market.  In 
order  to  make  a  nickeling  solution  the  pure  salt,  which  comes  in  green 
crystals,  is  obtained,  and  is  dissolved  in  hot  water  at  the  rate  of 
about  three  quarters  of  a  pound  to  a  gallon  of  water.  The  vat  used 
to  hold  a  nickeling  solution  is  usually  of  wood  lined  with  lead.  The 
joints  in  the  lead  lining  are  burned  together,  not  soldered.  In  the 
preparation  of  articles  for  nickel  plating,  they  must  be  very  carefully 
polished  and  cleaned  by  scouring,  dipping  in  a  hot  lye  solution,  and 
pickling  in  acid.  The  fatter  is  very  important  since  the  acid  takes 
off  from  the  artic^s  the  thin  covering  of  oxide  which  is  likely  to 
stick  to  iron,  copper  and  brass  and  which  the  nickel  solution  has 


97 


no  tendency  to  remove.  If  the  oxide  covering  is  not  removed  the 
nickel  plate  will  come  off,  or  strip  as  it  is  called,  while  the  articles 
are  being  finally  burnished.  The  final  proceSvSes  of  nickel  plating 
are  polishing  and  burnishing.  Articles  to  be  plated  with  nickel  are 
hung  in  the  liquid  or  bath  very  much  as  already  described  under  sil- 
ver plating.  When  a  number  of  small  articles  are  to  be  nickel  plated 
they  are  often  suspended  in  a  string  from  the  same  wire.  Fig.  71 
shows  the  manner  of  suspending  screws  in  the  nickel  bath,  and  Figs. 
72  and  73  show  the  manner  of  suspending  bicycle  spokes  and  chains. 
Fig.  74  shows  the  form  of  a  vat  generally  used  in  nickel  plating. 
Nickel  vats  are  generally  larger  than  those  used  in  silver  plating. 
It  is  particularly  important  that  no  organic  (non-metallic)  impurities 
be  allowed  in  a  nickel  bath,  as  these  ruin  the  quality  of  a  nickel 
deposit. 

Electrotyping  is  a  process  of  reproducing  type  and  wood  cuts,  by 
means  of  an  electroplating  of  copper,  which  is  used  in  nearly  all 
large  printing  establishments.  In  electrotyping,  an  impression  or 
mould  is  made  of  the  type  which  is  set  up  as  for  printing.  This 
mould  is  usually  made  in  wax  or  soft  paper  pulp  b^pressing  it  hard 
upon  the  type.  After  the  surface  of  the  mould  is  properly  trimmed 
up,  it  is  coated  with  fine  plumbago  or  some  similar  conductor  which 
is  carefully  brushed  over  it,  so  that  an  electrolytic  shell  of  copper 
may  be  deposited  upon  it.  The  plumbagoing,  as  it  is  called,  is 
necessary  because  the  mould  itself  is  a  non-conductor,  and  the 
current  which  is  necessary  to  make  an  electrolytic  coating  cannot  be 
sent  through  it.  Plumbago  is  powdered  graphite,  and  is  a  fairly 
good  conductor  (L,esson  i,  page  4),  so  that  a  thin  coating  brushed 
over  the  surface  of  the  mould  enables  it  to  conduct  the  current. 
Thus  prepared,  the  mould  is  hung  in  an  electrolytic  bath  consisting 
of  a  solution  of  sulphate  of  copper  (blue  vitriol)  to  which  a  small  per- 
centage of  sulphuric  acid  is  added.  The  anode  of  the  electrolytic 
cell  is  a  plate  of  copper,  and  the  cathode  is  the  mould.  TLo  thick- 
ness of  the  shell  of  copper  which  is  deposited  on  the  mould  varies 
from  that  of  the  sheet  of  paper  upon  which  this  lesson-is  printed  to 
several  times  that  thickness,  depending  upon  how  much  printing  is 
to  be  done  from  the  electrotyped  plates.  When  the  copper  deposit  is 
of  proper  thickness  the  mould  is  removed  from  the  bath,  and  the 
copper  shell  is  separated  from  the  mould.  The  shell  is  then  trimmed 
and  finally  "backed  up"  by  a  filling  of  type  metal  wnich  is  melted 
and  poured  upon  the  back  of  the  shell.  Electrotyped  plates  have  a 
great  advantage  over  type  in  having  a  permanent  form  and  in  wear- 
ing much  better.  As  soon  as  the  mould  for  electrotyping  is  taken  off 
from  set  up  type,  the  type  may  be  distributed  and  u^ed  again. 

Copper  plating  is  also  used  sometimes  to  give  a  bronze  finish  to 
iron  lamp  posts,  gas  fixtures,  etc.,  and  it  is  used  to  make  a  foundation 
coating  upon  iron  articles  which  it  is  desired  to  sliver  plate. 


98 


Plating  with  other  metals  than  those  referred  to  above,  such  as 
iron,  tin,  and  zinc,  is  sometimes  carried  out  for  special  purposes.  For 
this  purpose,  special  solutions  and  peculiar  care  in  handling  the  articles 
must  be  used.  It  is  even  possible  to  electrolytically  deposit  brass, 
german  silver,  or  other  alloys.  The  latter  requires  extreme  care, 
however,  in  the  management  of  the  solutions  and  the  regulation 
of  the  electric  pressure  and  current  supplied  to  the  vats. 

A  very  useful  application  of  electrometallurgy  is  the  refining  of 
copper.  In  this  operation  the  crude  copper  which  comes  from  ordinary 
smelting  works  with  from  two  to  five  per  cent  of  impurities,  is  re- 
fined by  electrolysis  so  that  only  very  minute  amounts  of  impurities 
remain.  We  have  already  seen  (Lesson  7,  page  45)  the  effect  of  im- 
purities in  reducing  the  conductivity  of  copper  and  other  metals. 
When  the  electrolytic  method  of  refining  copper  is  properly  carried 
out,  it  leaves  such  a  small  amount  of  impurities  that  the  electrolytic 
copper  has  almost  as  great  conductivity  as  pure  copper.  Copper 
wires  to  be  used  in  electric  lighting  and  in  the  manufacture  of  elec- 
tric machines  are  therefore  generally  made  of  electrolytic  copper.  It 
is  usual  for  sucr  wires  to  have  more  than  96  per  cent  of  the  conduct- 
ivity of  pure  copper.  The  small  amount  of  impurities  which  do 
remain  in  electrolytically  refined  copper  is  largely  composed  of  silver 
and  iron.  In  electrolytic  refining  the  crude  copper  is  cast  into  heavy 
plates  which  are  used  as  anodes  in  depositing  vats,  the  solution  in 
which  is  copper  sulphate  with  a  little  sulphuric  acid.  The  cathodes 
at  first  are  sheets  of  pure  copper,  but  they  grow  by  deposition  into 
thick  plates  of  copper  which  may  be  worked  into  bars  and  drawn 
into  wires  as  desired.  The  action  in  the  depositing  vats  is  quite 
similar  to  that  which  goes  on  in  a  copper  voltameter.  In  copper  re- 
fining works  enormous  dynamos  are  used,  and  a  great  number  of 
tanks  or  vats  each  containing  a  number  of  anodes  and  cathodes  ar- 
ranged alternately  and  connected  in  parallel  are  provided.  The  vats  are 
ordinarrly  connected  in  series;  or  sets  of  a  number  of  vats  connected  in 
series  are  connected  in  parallel.  The  pressure  required  to  pass  the  cur- 
rent through  each  vat  is  quite  small  and  consequently  a  number  of  vats 
may  be  connected  in  series  without  causing  the  total  pressure  to  ex- 
ceed 100  volts.  It  is  desirable  that  the  pressure  required  at  each  vat 
be  as  little  as  possible  in  order  to  avoid  the  deposition  of  impurities 
on  the  cathodes  and  also  to  save  power.  The  power  used  in  each 
vat  is  equal  to  the  difference  of  pressure  between  the  anodes  and 
cathodes  multiplied  by  the  current  flowing  through  the  vat.  It  is 
desirable  to  have  as  great  a  current  flow  as  will  give  a  fairly  smooth 
deposit,  in  order  that  the  time  required  in  depositing  each  pound  of 
copper  may  be  as  small  as  possible.  Any  reduction  made  in  the  cur- 
rent without  changing  the  pressure  simply  reduces  in  a  proportional 
rate  the  amount  of  copper  deposited  so  that  the  power  required  to 
deposit  a  pound  of  copper  is  not  materially  changed.  If  the  pressure 


QQ 


FIG.  75. 

required  to  pass  the  current  through  the  vats  be  reduced  without 
changing  the  current  it  at  once  reduces  the  power  required  to  de- 
posit a  pound  of  copper  and  a  saving  in  the  cost  of  manufacture  is 
effected.  In  order  to  reduce  the  pressure,  the  anodes  and  cathodes 
are  set  as  closely  together  as  possible  without  interfering  with  the 
circulation  of  the  solution. 

During  the  process  of  refining  copper  by  this  means,  the  impuri- 
ties of  the  crude  copper  are  mostly  dissolved  in  the  solution  or  are 
thrown  to  the  bottom  of  the  vats  as  mud  or  sludge.  Copper  that  is 
to  be  electrolytically  refined,  usually  contains  some  silver  and  a  little 
gold.  During  the  refining  process  these  form  salts  which  form  part 
of  the  mud  and  the  precious  metals  are  recovered  by  the  ordinary 
method  of  smelting  when  the  mud  is  removed  from  the  vats  from 
time  to  time.  Iron  and  lead  are  also  contained  in  the  crude  copper, 
as  are  small  quantities  of  other  metals.  The  lead  goes  to  the 
bottom  of  the  tank  like  the  gold  and  silver,  and  the  iron  dissolves  in 
the  solution  but  is  not  deposited  on  the  cathode  except  in  very  small 
amounts  unless  the  pressure  at  the  vat  is  too  high.  Electrolytic 
refining  of  crude  copper  from  ores  which  contain  silver  is  particularly 
useful  because  it  is  the  cheapest  method  of  separating  the  silver  from 
the  copper.  Great  electrolytic  refineries  have,  therefore,  been  erected 
at  the  Montana  copper  mines,  the  ores  of  which  usually  contain  a 
valuable  portion  of  silver. 

Electrolysis  has  been  applied  to  the  refining  of  other  metals  but 
without  great  success.  It  has  also  been  used  in  the  recovery  of  pre- 
cious metals  from  their  ores,  but  without  success,  although  it  may  yet 
prove  its  value  in  working  certain  kinds  of  ores. 

The  electric  arc,  such  as  is  seen  in  arc  lamps  but  much  larger, 
has  been  successfully  used  in  working  ores,  especially  in  producing 
the  metal  aluminum.  This  application  is  usually  called  electric 


100 


smelting.  The  action  which  occurs  in  electric  smelting  is  partially 
due  to  chemical  action  set  up  by  the  intense  heat  cf  the  electric  arc 
which  melts  the  ores,  and  partially  due  to  the  electrolysis  caused  by 
the  current  passing  through  the  ores  in  their  melted  state.  It  is  by 
electric  smelting  from  corundum  and  similar  material  that  the  Cowles 
Company  of  Lockport,  N.  Y.,make  the  aluminum  which  is  found  in 
their  well-known  aluminum  bronze.  It  is  also  by  the  electric  smelt- 
ing process  that  the  Pittsburg  Reduction  Company  make  aluminum. 
Fig.  75  shows  a  set  of  electric  furnaces  which  are  used  in  England 
for  the  production  of  aluminum  bronze.  The  current  for  these  is 
furnished  by  a  dynamo  of  several  hundred  horse-power. 

Copyrighted,  1894, 


The  National  School  of  Electricity. 

REVIEW  OF  LESSON  XIII. 

Points  for  Review.     1.     What  is  electroplating? 

2.  What  metals  are  most  commonly  used  in  plating? 

3.  What  is  a  salt  of  a  metal? 

4.  What  is  nitrate  of  silver?      What   is  cyanide  of  silver?      What  is  cyanide  of 
potassium? 

5.  How  may  nitrate  of  silver  be  made? 

6.  Why  is  a  solution  of  nitrate  of  silver  not  used  for  silver  plating?     What  is  the 
best  solution  to  use  for  silv.er  plating? 

'7.     How  may  cyanide  of  silver  be  made? 

8.  What  kinds  of  vats  are  usually  used  in  silver  plating? 

9.  Why  must  the  anodes  in  a  plating  vat  be  of  the  same  metal  as  the  plate  which 
is  deposited? 

10.  Upon  what  does  the  quality  of  the  electrolytic  deposit  of  a  metal  depend? 

11.  What  is  the  effect  on  a  silver  deposit  when  the  current   is  too  great  and  when 
it  is  too  small? 

12.  What  is  the  object  of   cleaning  articles  before  placing  them  in  the  plating  bath? 
How  is  the  cleaning  done?. 

13.  How  are  the  insides  of  silver  articles  gilded? 

14.  Upon  what  base  metals  is  nickel  usually  plated? 

15.  Why  is  nickel  used  instead  of  silver  for  finishing    articles  which   will  receive 
severe  usage? 

16.  What  solution  is  used  for  nickel  plating? 

17.  What  effect  on  nickel  plating  is  caused  by  the  presence  of  organic  impurities  in 
the  bath? 

18.  For  what  purpose  is  electrotyping  used? 

19.  Why  are  the  ordinary  electrotype  molds  plumbagoed? 

20.  How  is  an  electrotype  finished  after  the  copper  shell  has  been  deposited? 

21.  Upon  what  does  the  thickness  of  the  copper  shell  of  an  electrotype  depend? 

22.  What  are  the  advantages  of  electrolytic  refining  of  copper? 

23.  What  solution  is  used  in  electrolytic  copper  refining?     In  what  form  does  the 
copper  come  to  the  electrolytic  refining? 

24.  Why  is  electrolytic  copper  preferred  for  wire  which  is  used  in  the  electrical 
industries? 


XIV. 

THE  ELECTRIC  TELEGRAPH. 

The  electric  telegraph,  as  it  is  commonly  used,  is  to  a  large 
degree  a  growth  from  the  discoveries  of  Prof.  Joseph  Henry  of 
Princeton  College.  Henry's  discoveries  were  directly  applied  to 
telegraphy  by  S.  F.  B.  Morse,  an  American  artist  and  inventor, 
whose  fame  rests  principally  upon  his  telegraphic  inventions. 
Before  the  inventions  of  Morse,  many  means  of  communicating 
over  considerable  distances  had  been  tried.  In  some  of  these 
electricity  played  a  part,  but  the  essential  feature  of  the  tele- 
graphs of  the  present  day,  the  electromagnet,  was  not  applied 


FIG.  76. 


POSITION  AND  MOVEMENT. 
FIG.  77. 

with  success  until  Morse's  inventions.  Indeed,  Morse's  inventions 
came  very  soon  after  the  discovery  of  the  essential  property  of  the 
electromagnet,  that  of  losing  its  magnetism  upon  the  interruption  of 
the  magnetizing  current.  (Lesson  6,  page  39).  The  Morse  system  of 
telegraphy  has  four  elements  connected  in  series:  ist,  a  battery  or 
other  source  of  an  electric  current;  2nd,  a  key  by  means  of  which  the 
electric  circuit  may  be  made  and  broken  to  produce  signals;  3rd,  a 
line  of  wire  running  from  the  point  at  which  the  signals  are  pro- 
duced to  the  point  where  the  signals  are  to  be  received;  4th,  an 
electromagnetic  sounder  or  register  by  means  of  which  the  signals 
may  be  distinguished  or  recorded. 

The  battery  ordinarily  used  in  telegraphy  is  the  common  gravity 
form  described  in  lesson  4,  but  galvanic  batteries  have  been  widely 
replaced  in  telegraph  service  by  small  dynamos  during  the  past  few 
years. 

The  ordinary  form  of  telegraph  key  is  shown  in  Fig.  76.  This 
consists  of  a  lever,  which  is  pivoted  so  that  it  may  be  moved  through 
a  small  vertical  range  by  pressing  the  fingers  upon  the  button  at  its 
end  (the  left  hand  of  the  figure).  The  spring  shown  at  the  center  of 
the  figure  tends  to  keep  the  lever  at  the  upper  end  of  its  stroke,  so 
that  an  operator,  in  making  signals  with  the  key,  need  only  depress 
the  lever  and  it  will  return  to  its  normal  position  upon  removing  the 
pressure.  The  operator's  fingers  are  therefore  placed  on  the  button 
in  the  way  shown  in  Fig.  77.  The  left-hand  leg  of  the  key  is  con- 
nected to  a  contact  point  which  is  seen  directly  above  the  leg  and 


103 


FIG.  78. 

which  is  insulated  from  the  frame.  The  lever  carries  a  correspond- 
ing contact  point  directly  above  the  insulated  one.  The  upper 
contact  point  is  in  electrical  contact  with  the  right  hand  leg 
through  the  metal  of  the  lever  and  frame.  When  the  key  is  con- 
nected into  a  circuit  by  cutting  the  circuit  wire  and  attaching  the 
two  ends  to  the  two  legs  of  the  key,  the  circuit  may  be  made  and 
broken  at  the  will  of  the  operator  by  depressing  or  raising  the  lever. 
Fig.  78  shows  a  key  without  legs  but  with  binding  posts  for  the  con- 
nection of  the  wires.  As  ordinarily  arranged,  a  telegraph  circuit  is 
broken  only  at  the  time  of  making  signals,  consequently  a  switch  is 
placed  on  the  key  so  that  the  circuit  can  be  closed  when  the  key  is 
not  in  use.  The  handle  of  the  switch  is  sLown  in  the  figure  just  to 
the  left  of  the  contacts.  With  this  arrangement  of  the  circuit  it 
is  possible  to  place  a  number  of  stations  in  series  on  one  line  (Fig. 
79)  and  since  the  circuit  is  normally  complete — that  is,  it  is  always 
complete  when  not  in  use — the  operator  at  any  station  may  signal 
any  other  at  any  time,  provided  no  other  operator  is  using  the  line. 

The  current  used  in  telegraphy  is  quite  small — it  does  not  often 
exceed  fifty  milliamperes — and  therefore  it  is  possible  to  satisfactorily 
use  the  earth  for  one  side  of  the  circuit.  A  telegraph  line  therefore 
ordinarily  consists  of  a  wire  supported  on  wooden  poles  and  running 


FIG.  79. 

from  station  to  station.  At  its  ends  the  wire  is  connected  to  the  earth 
by  means  of  ground  plates,  as  shown  in  Fig.  79.  The  wire  used  is 
generally  number  6  or  8  iron  wire  gauge  wire  made  of  the  best  gal- 
vanized iron,  but  for  some  short  lines  steel  wire  is  used  and  for  some 
of  the  most  important  lines  between  large  cities,  copper  wire  is  used. 
Wires  as  large  as  number  4  and  as  small  as  number  10  are  sometimes 
used.  The  choice  of  the  size  and  kind  of  wire  depends  largely  upon  the 
length  and  importance  of  the  line,  upon  which  depends  the  amount 


FIG.  80. 

oi  battery  power  which  is  necessary  to  operate  the  signals  and  the 
care  with  which  the  line  is  kept  in  good  condition. 

In  the  earlier  days  of  the  Morse  telegraph  it  was  thought  neces- 
sary to  receive  the  signals  constituting  a  telegraphic  message  in  a 
permanent  form  by  means  of  a  recording  register.  Such  a  register 
is  shown  in  Fig.  80.  This  consists  of  a  case  containing  a  horse  shoe 
electromagnet,  the  windings  of  which  are  connected  in  series  with 
the  telegraph  circuit.  Over  the  poles  of  the  magnet  is  an  armature 
of  soft  iron  which  is  held  against  a  stop  by  the  pull  of  the  magnet 
when  the  current  flows  through  the  circuit.  When  the  current  is 
interrupted  by  means  of  a  key,  as  in  sending  signals,  the  electro- 
magnet loses  its  magnetism  and  the  armature  is  no  longer  attracted, 
so  that  a  small  spring  which  is  attached  to  it,  is  able  to  pull  it  back 
from  the  stop.  Thus,  as  current  impulses  are  sent  along,  the  line  by 
making  and  breaking  the  circuit  at  a  key,  the  pulls  of  the  magnet 
and  of  the  spring  alternately  draw  the  armature  forwards  and  back- 
wards. The  movement  of  the  armature  is  recorded  or  registered  by 
means  of  a  pen  or  a  blunt  point  on  a  narrow  strip  of  paper  which  is 
automatically  fed  from  the  roll  shown  in  the  figure.  This  paper 
tracing  of  the  signals  may  be  read  by  the  receiving  operator  and 
translated  into  ordinary  language  upon  a  telegraph  blank,  and  then 
delivered  to  the  person  for  whom  the  message  is  intended. 

Telegraphic  signals  are  made  up  of  a  combination  of  long  and 
short  current  impulses,  which  are  made  by  pressing  the  sending  key 
at  proper  intervals  and  for  proper  periods,  and  which  are  recorded  on 
a  register  as  long  and  short  dashes.  Each  combination  of  dashes 
represents  a  letter  of  the  alphabet  or  a  certain  much  used  word  or 


105 


phrase.     The  Morse  Alphabet,  as  it  is  called,  which  is  used  in  this 
country,  is  given  below.     (Fig.  81). 

Morse  Alphabet. 

A                  B  C                   D  E  P  G 

HI  J                    K  L  M  N 

OP  Q                   R  S  T  U 

V                   W  X  Y  Z  & 

,     NUMERALS. 
1  2  3  4  5 

6789  0 


PUNCTUATION  MARKS 
Period.  Comma.  Semi-colon  Colon 


Quotation  mark.  Parenthesis.  Interrogation, 

Paragraph.  Exclamation.  Dollar  mark 

As  the  Morse  telegraph  came  into  considerable  use,  the  opera- 
tors found  that  they  could  read  the  signals  passing  over  the  line  by 
listening  to  the  clicks  of  the  register  armature  as  it  moved  back  and 
forth  between  its  stops  under  the  influence  of  the  current  impulses. 
The  paper  roll  was  therefore. abandoned,  as  "  reading  by  sound  "  was 
quicker  and  more  convenient  than  translating  the  message  from  the 
paper  tracing  of  the  signals.  To  make  reading  by  sound  as  easy  as 
possible  the  working  mechanism  of  the  register  was  altered  into  that 
of  the  sounder  (Fig.  82).  The  figure  plainly  shows  the  arrangement 
ot  the  sounder.  The  armature  has  attached  to  it  a  substantial  brass 
bar.  This  bar  is  pivoted  at  its  right  hand  end,  as  shown  in  the 
figure,  so  that  its  left  hand  end  may  move  up  and  down  between  ad- 
justable stops,  as  shown.  To  the  right  hand  end  of  the  bar  is 
attached  a  spring  which  draws  the  bar  against  the  upper  stop  when 
no  current  is  flowing  in  the  magnet.  The  black  cylinders  shown 
about  the  center  of  the  figure,  compose  the  magnet.  This  magnet 
consists  of  two  cores  of  iron  about  three-eighths  of  an  inch  in  diame- 
ter and  one  inch  and  a  half  long,  wound  with  wire  and  covered  with 
black  paper  or  a  short  piece  of  hard  rubber  tube.  The  cores  are 
screwed  fast  to  an  iron  base  so  as  to  make  a  horse-shoe  electromagnet. 
The  armature  is  shown  at  the  top  of  the  figure,  above  the  magnet. 
When  current  flows  in  the  magnet  winding,  the  armature  is  attracted 


toe 


FIG.  83. 

and  the  bar  drawn  against  the  lower  stop.  As  the  bar  moves  back 
and  forth  it  makes  a  sharp  click  when  it  strikes  one  of  the  stops. 
The  strength  of  the  spring  is  adjustable  by  means  of  a  screw  so  that 
the  sounder  may  be  adjusted  for  use  within  a  certain  range  of  cur- 
rents of  different  strength.  To  successfully  read  signals  from  a 
sounder  much  experience  is  necessary,  but  operators  become  very 
expert  by  long  practice.  It  is  necessary  in  reading  to  distinguish 
between  the  clicks  of  the  armature  against  the  top  and  bottom  stops. 
A  little  consideration  will  show  that  the  length  of  time  between  the 
clicks  when  the  armature  strikes  the  bottom  stop  and  when  it  strikes 
the  top  stop  distinguishes  between  dots  and  dashes,  since  the  dots 
and  dashes  represent  intervals  during  which  current  is  flowing 
through  the  magnet.  The  interval  of  time  between  the  top  click  and 
the  bottom  click  represents  the  spacing  between  letters  and  words, 
because  the  spacing  represents  intervals  during  which  no  current 
flows,  or  during  which  the  signal  key  is  open. 

Telegraph  sounders  require  only  a  fraction  of  an  ampere  to  oper- 
ate them,   but    to    cause    that    fraction  to  flow  through  a  long   line 

107 


KEY. 


LOCAL  BATTERY 

FIG.  84. 

which  necessarily  has  a  high  resistance,  requires  the  use  of  a  battery 
of  a  very  large  number  of  cells.  This  is  undesirable  because  the 
cells  are  expensive  to  buy  and  to  keep  up.  Long  telegraph  lines  are 
therefore  furnished  with  instruments  which  operate  like  sounders, 
but  which  are  made  very  sensitive  by  placing  a  great  many  turns  of 
fine  wire  on  their  magnets,  so  that  they  may  even  be  satisfactorily 
operated  on  eight  or  ten  milliamperes.  These  instruments  are  called 
relays  (Fig.  83).  Reading  signals  directly  from  a  relay  is  not  usually 
attempted  as  the  motion  of  its  armature  is  so  delicate  that  it  makes 
very  little  noise,  but  the  armature  and  one  of  its  stops  are  arranged 
as  part  of  a  local  circuit  which  contains  a  sounder  and  a  couple  of 
gravity  cells.  (Fig.  84.)  As  the  relay  armature  moves  back  and 
forth  it  makes  and  breaks  the  local  circuit  and  reproduces  in  it  the 
signals  which  pass  over  the  main  line.  The  sounder  in  the  local 
circuit  gives  the  signals  exactly  as  they  pass  over  the  line.  Fig.  85 
shows  a  set  consisting  of  key,  relay,  and  sounder  on  a  common  base. 
To  still  further  economize  in  long  and  important  lines  arrange- 
ments are  made  to  send  more  than  one  message  at  a  time  over  each 
wire.  When  a  telegraph  wire  is  arranged  so  that  two  messages  may 
be  transmitted  over  the  wire  at  once,  one  being  sent  from  each  end, 
the  wire  is  said  to  be  duplexed.  When  it  is  so  arranged  that  both 

308 


taei 


FIG.  85. 

messages  may  be  sent  from  one  end,  the  wire  is  usually  said  to  be 
diplexed.  Diplexed  wires  are  not  ordinarily  used,  except  in  com- 
bination. When  a  wire  is  arranged  so  that  four  messages  may  be 
transmitted  over  it  at  once,  two  being  sent  from  each  end,  it  is  said 
to  be  quadruplexed.  In  arranging  a  quadruplex,  a  combination  is 
practically  made  of  a  duplex  and  a  diplex  arrangement.  These  ar- 
rangements for  multiple  telegraphy,  as  it  is  called,  will  be  explained 
in  the  next  lesson. 

Copyrighted,  1894, 


109 


The  National  School  of  Electricity. 

REVIEW  OF  LESSON  XIV. 

Points  for  Review :     1.     What  are  the  eosential  elements  of  an  electric  telegraph? 

2.  What  is  the  normal  condition  of  a  telegraph  circuit? 

3.  How  are  telegraphic  signals  made?     How  are  they  received? 

4.  What  kind  of  wire  is  used  for  telegraph  lines? 

5.  How  are  letters  and  words  made  out  of  telegraphic  signals? 

6.  What  is  a  sounder?     What  is  a  relay? 

7.  Why  are  relays  used  in  telegraphing? 

8.  What  is  a  "local  circuit?" 

9.  What  is  the  object  of  multiple  telegraphy? 

10.     What  is  duplex  telegraphy?      What  is  diplex  telegraphy?      What  is  quadruplex 
telegraphy? 

LESSON  xv. 

MULTIPLE  TELEGRAPHY. 

The  commonest  arrangement  for  duplex  telegraphy  requires  a 
special  relay  which  is  connected  as  shown  in  Fig.  86.  It  is  seen  that 
three  points  of  connection  are  made  to  the  wire  which  is  wound  on 
the  electromagnet  of  the  relay,  one  point  at  each  end  of  the  wire  and 
a  third  at  the  middle  of  the  wire.  The  wire  is  wound  in  the  usual 
way  with  one-half  on  each  leg  of  the  electromagnet.  If  a  current  be 
passed  through  the  wire  from  one  end  to  the  other,  the  relay  will 
act  exactly  as  a  common  relay.  If  a  current  be  sent  into  the  relay 
from  the  middle  point,  the  current  will  divide,  and  the  two  parts  will 
pass  through  the  windings  on  the  two  legs  of  the  electromagnet  in 
such  a  way  that  their  magnetic  effects  are  in  opposite  directions.  If 
the  two  divisions  of  the  current  are  made  equal,  their  opposite 
magnetic  effects  are  equal  and  exactly  neutralize  each  other,  so  that 
the  core  and  amature  of  the  relay  are  not  affected.  Such  a  relay  is 
called  a  differential  relay.  The  exact  arrangement  of  the  windings 
on  a  differential  relay  may  vary  considerably,  but  the  purpose  and 
effect  of  all  arrangements  are  exactly  the  same  as  described.  Fig.  87 
is  a  diagram  of  the  connections  made  at  a  telegraph  station  for 
duplex  telegraphy  using  a  differential  relay.  This  arrangement  is 
often  called  the  differential  duplex.  The  figure  shows  by  the  arrow- 
heads that  the  current  sent  into  the  line  at  the  station  divides  in  the 
relay  belonging  to  that  station  and  half  of  it  passes  to  the  ground 
through  the  resistance  R  and  back  to  the  battery.  The  other  half 
of  the  current  goes  through  the  line  and  the  relay  ot  the  station  for 
which  the  signals  are  intended,  and  returns  to  the  battery  by  way  of 
the  earth.  The  two  halves  of  the  current  pass  through  the  coils  of 
the  home  relay  in  opposite  directions  and  neutralize  each  other's 
magnetic  effects.  A  message  sent  from  the  home  key  therefore  does 

no 


B,  main  battery.     K,   Key.     M,  differential  relay.      S,  sounder. 

L1  and  L2,  local  batteries.     N,  D,  transmitter. 

X8  and  X3,  special  resistances. 

not  affect  the  home  relay.  That  part  of  the  current  which  goes  into 
the  line  passes  through  the  winding  of  the  distant  relay  in  the  usual 
manner  so  as  to  make  a  signal.  In  this  way  the  operator  at  any 
station  on  a  line  can  signal  another  without  affecting  his  home  relay. 
Two  messages  can  therefore  be  transmitted  at  the  same  time  in 
opposite  directions  between  two  stations  without  interference.  This, 
of  course,  requires  two  operators^  one  to  send  and  one  to  receive  the 
message  at  each  station.  The  figure  shows  a  special  device  called 
a  transmitter,  for  making  the  signals.  This  is  explained  on  page  114. 
In  order  that  a  differential  duplex  may  work,  it  is  absolutely 
necessary  that  the  current  in  the  relay  divide  quite  accurately  into 
halves.  This  is  effected  by  properly  adjusting  the  resistance  of  the 
rheostat  R,  in  the  home  branch  of  the  circuit.  The  branch  of  the 
circuit  at  each  -station  containing  the  resistance  R,  is  called  the  arti- 
ficial line,  since  it  is  made  to  represent  as  far  as  possible  the  condition 
of  the  actual  line  in  order  to  make  the  duplex  work  satisfactorily. 
The  resistance  of  the  line  is  easily  balanced  by  making  the  resistance 
R  so  that  it  may  be  adjusted  by  plugs  to  suit  the  condition  of  the 
line  (Fig.  88.)  Certain  smaller  resistances  (X2  and  X3  in  the  figure), 
are  also  used  in  the  home  circuits  to  smooth  the  action  of  the  trans- 
mitter in  making  and  breaking  the  circuit.  The  electrostatic  capa- 
city of  the  line  affects  the  rise  and  fall  of  the  current  in  it  as  the  sig- 
nals are  transmitted,  and  in  order  to  get  the  best  results  with  the 
differential  relay  the  artificial  line  is  arranged  with  a  condenser,  C, 
connected  in  parallel  with  the  resistance  R,  the  capacity  of  which 
balances  that  of  the  line.  This  condenser  is  usually  made  of  sheets 
of  tin  foil  insulated  by  sheets  of  thin  mica  or  paraffined  linen  paper, 
and  the  capacity  connected  into  circuit  may  be  adjusted  by  means  of 
plugs  (Fig.  89).  When  a  telegraph  circuit  is  arranged  for  duplex 
working  it  is  necessary  to  have  a  line  battery  at  each  of  the  stations 

111 


n 

FIG.  91 


FIG. 


For  simple  working  all  the  battery  may  be  placed,  if  desired,  at  a  sin 
gle  point  along  the  line. 

When  a  line  is  arranged  for  diplex  working,  two  keys  are  placed 
at  the  sending  station  and  two  relays  are  placed  at  the  receiving 
station.  One  of  these  relays  called  the  polarized  relay,  has  a  perma- 
nently magnetized  steel  armature.  When  the  armature,  which  lies 
across  the  poles  of  a  horseshoe  electromagnet,  is  permanently  mag- 
netized, or  polarized,  it  is  attracted  when  the  current  flows  in  one 
direction  and  repelled  when  the  current  flows  in  the  opposite  direc- 
tion. Advantage  may  best  be  taken  of  this  by  placing  the  polarized 
armature  between  the  poles  of  the  electromagnet,  as  in  Figs.  90  and 
91.  The  end  of  the  armature  will  then  stick  against  either  pole 
indifferently  when  no  current  flows,  if  it  is  not  restrained  by  a  spring. 
When  a  current  flows  in  one  direction  the  end  of  the  armature  will 
move  up  to  one  pole,  and  when  the  current  is  reversed  the  armature 
will  move  over  to  the  other  pole,  as  shown  in  the  figures.  A  polar- 
ized relay  made  upon  this  principle  may  be  operated  by  signals 
which  are  given  by  reversing  the  current  in  the  circuit  instead  of 
making  and  breaking  the  circuit  as  in  simple  telegraphy.  Fig.  92 
shows  a  common  form  of  polarized  relay  in  which  the  armature  is 
kept  polarized  by  means  of  a  strong  permanent  magnet  to  which  it  is 
attached.  A  key  for  sending  signals  by  reversing  the  current  is 
called  a  pole  changer. 


112 


FIG.  94. 


113 


GROUND  CS3  GROUND 

FIG.  95. 

B1,  small  battery.    B2,  large  battery,    K1,  pole  changer  and  key.     K2,  transmitter 
and  key.     R1,  polarized  relay.     Ra,  neutral  relay. 

It  is  possible  to  send  signals  to  a  common  or  neutral  relay  over 
the  same  line  as  that  used  with  the  polarized  relay  without  interfer- 
ing with  the  action  of  the  latter.  In  order  that  the  currents  which 
actuate  the  polarized  relay  shall  not  also  work  the  neutral  relay,  the 
latter  is  adjusted  to  respond  only  to  a  current  which  is  greater  than 
that  required  to  actuate  the  polarized  relay.  Hence  the  operation  of 
the  diplex  arrangement  depends  upon  the  use  of  currents  of  two 
strengths.  One  of  these  currents  is  quite  weak  and  is  reversed  in 
sending  signals,  so  that  a  polarized  relay  is  used  with  it;  the  other 
current  is  stronger  and  is  increased  and  decreased  by  the  sending  key 
in  sending  signals  to  the  neutral  relay,  instead  of  making  and  break- 
ing the  circuit,  since  doing  the  latter  would  interfere  with  the  signals 
sent  to  the  polarized  relay.  A  key  arranged  to  increase  and  decrease 
the  current  in  sending  signals  is  usually  called  a  transmitter.  The 
increase  and  decrease  of  the  current  is  gained  by  alternately  connect- 
ing into  circuit  a  large  and  a  small  battery  (Fig.  95).  In  order  that 
common  telegraph  keys  may  be  used  by  the  operators  in  sending 
messages  by  the  diplex  arrangement  it  is  usual  to  work  the  pole 
changer  and  transmitter  by  means  of  electromagnets  like  sounders, 
connected  individually  in  local  circuits  in  series  with  the  sending 
keys.  Figs.  93  and  94  show  common  forms  of  pole  changer  and 
transmitter.  Fig.  95  is  a  diagram  of  the  connections  at  the  sending 
and  receiving  stations  upon  a  diplex  telegraph  line. 

The  commonly  used  quadruplex  is  essentially  a  combination  of 
the  differential  duplex  and  the  diplex  which  have  just  been  explained. 
A  diagram  of  the  circuits  at  a  quadruplex  station  is  shown  in  Fig. 
96.  It  is  to  be  seen  in  this  that  the  polarised  and  neutral  relays  of 
the  diplex  arrangemetU  are  used,  but  they  are  wound  in  differential 
fashion.  This  makes  it  possible  to  send  two  messages  from  a  sta- 
tion without  interference  with  each  other  (diplex),  and  at  the  same 
time  without  interfering  with  the  receiving  of  two  messages  by  the 
differential  instruments  at  the  same  station.  The  key  arrangements  for 
the  quadruplex  are  the  same  as  those  of  the  diplex,  so  that  a  pole  chang- 
er and  transmitter  are  used,  though  for  simplicity  they  are  not  shown 


114 


»1<  Jl* 


FiG.  96. 

D1,  D*,  dynamos.     K1,  pole  changer  and  key.     K2,  transmitter  and  key. 

R1,  polarized  differential  relay,  R2,  neutral  differential  relay. 

X  and  x,  adjustable  resistances.     C,  condenser. 

in  the  figure.  The  figure  shows  the  use  of  dynamos  in  the  place  of 
batteries.  The  coils  marked  600  and  900  are  resistances  which  take 
the  place  of  the  resistances  X2  and  X3  of  Fig.  87.  The  coil  marked 
1 200  is  used  to  reduce  the  current  in  making  signals  for  the  neutral 
relay  and  fills  the  purpose  of  the  division  of  the  battery  into  a  large 
and  a  small  section  (page  114).  The  figures  represent  the  resistances  of 
the  coils  in  ohms.  The  signals  for  the  neutral  relay  are  made  by 
alternately  cutting  this  large  resistance  into  and  out  of  the  circuit 
by  the  transmitter.  The  action  can  be  understood  by  an  examina- 
tion of  the  figure.  For  satisfactory  quadruplex  working  the  artifi- 
cial line  must  be  kept  well  adjusted,  or  trouble  is  experienced  from 
blurring  the  signals  in  the  differential  instruments. 

Another  form  of  duplex  which  depends  for  its  operation  on  a 
balance  similar  to  that  of  a  Whcatstone  bridge,  is  often  used  on 
ocean  cables.  The  arrangement  is  shown  in  Fig.  97.  In  this  the 
relay  is  located  at  M.  In  the  arms  of  the  triangle,  R  A  and  R  B 
are  fixed  resistances,  and  P  is  a  variable  resistance.  Now,  according 
to  the  principle  of  the  Wheatstone  bridge,  if  the  resistance  in  R  A 
is  to  the  resistance  in  R  B  as  the  resistance  of  the  line  is  to  the 


115 


GROUND      (3  3      GROlWfr 

FIG.  98. 

K1  and  K2,  pole  changers  with  keys.     R1  and  R2,  polarized  differential  relays. 

B1  and  B2,  batteries. 

resistance  P,  no  current  will  flow  through  the  relay  M  from  the  home 
battery.  Current  from  a  distant  battery  will,  however,  work  the 
relay,  since  it  will  come  in  from  the  line  to  A,  where  it  will  divide, 
part  going  around  through  A  R  B  in  it,  path  to  earth,  but  the 
greater  part  passing  through  the  lower  resistance  of  the  relay. 

A  duplex  may  also  be  operated  by  means  of  differential  polar- 
ized relays  (Fig.  98).  This  is  practically  the  same  in  operation  as 
the  differential  duplex  already  explaJ  ied,  but  pole  changers  are  used 
to  send  the  signals,  and  polarized  relay:  are  used  to  receive  them. 

Various  plans  for  sending  more  than  four  messages  over  one 
wire  have  been  devised,  but  they  have  been  too  complicated  to  be 
successful  in  operation  and  can  receive  no  attention  here.  Where 
more  than  four  messages  arc  sent  over  a  wire,  the  arrangement  is 
ordinarily  called  multiplex  telegraphy. 

Methods  have  also  been  devised  for  sending  messages  by 
machines  instead  of  by  hand.  Such  machines  are  used  to  a  con- 
siderable extent  for  special  work,  such  as  sending  press  despatches, 
stock  quotations,  etc.  It  is  usual  for  machine-sent  messages  to  be 
received  by  special  machine  recorders,  which  print  the  messages 
either  in  Morse  characters  or  directly  in  the  English  alphabet. 
Where  machines  are  used  in  telegraphy  the  arrangement  is  ordi- 


narily  spoken  of  as  automatic  telegraphy. 

Devices  have  also  been  invented  by  means  of  which  a  written 
message  or  sketch  may  be  transmitted  exactly  as  it  is  written  or 
drawn.  These  are  ordinarily  spoken  of  as  devices  for  autographic 
telegraphy.  The  most  successful  of  these  arrangements  is  the  Gray's 
telautograph,  but  none  of  them  have  yet  come  into  common  use  on 
account  of  their  complications. 

Copyrighted,  1894, 


The  National  School  of  Electricity. 

REVIEW  OF  LESSON  XV. 

Points  for  Review:     1.     What  is  a  differential  relay?     For  what  purpose  is  it  used? 

2.  What  is  the  principle  of  the  operation  of  the  differential  duplex  ? 

3.  What  is  a  polarized  relay  ?     For  what  purpose  is  it  used  ? 

4.  What  is  a  pole  changer?     For  what  purpose  is  it  used  ? 

5.  What  is  the  principle  of  the  operation  of  the  diplex  ? 

6.  What  is  the  principle  of  the  operation  of  the  quadruplex  ? 

7.  What  is  the  principle  of  the  operation  of  the  bridge  duplex  ? 

8.  Why  is  an  artificial  line  used  in  duplex  and  quadruplex  telegraphy  ? 

9.  What  is  autographic  telegraphy  ? 


XVI. 

THE  TELEPHONE. 

Unlike  telegraphy,  which  is  the  oldest  commercial  application 
of  electricity,  the  telephone  is  one  of  the  later  commercial  applica- 
tions. The  word  telegraph  comes  from  two  Greek  words  which 
mean,  when  combined,  to.  write  at  a  distance,  while  telephone  comes 
from  two  Greek  words  which  mean  to  speak  at  a  distance.  The  first 
telephone  that  can  be  given  the  credit  of  commercial  success,  was 
invented  by  Alexander  Graham  Bell,  and  was  privately  exhibited  by 
him  at  the  Centennial  Exposition  at  Philadelphia,  in  1876.  Dr. 
Elisha  Gray  applied  for  patents  on  practically  the  same  telephone 
mechanism  at  the  same  time,  and  the  simultaneous  invention  of  the 
instrument  by  the  two  noted  men  gave  rise  to  one  of  the  famous  pat- 
ent law  suits  of  the  age.  Since  that  time  the  usefulness  of  this  device 
has  been  greatly  increased  by  other  inventions  which  make  its  service 
more  perfect.  The  telephone  in  its  improved  form  has  added  wonder- 
fully to  the  ease  and  quickness  with  which  many  kinds  of  business 
may  be  transacted,  and  it  may  be  said  to  have  almost  revolutionized 
some  of  the  methods  of  doing  business. 

The  telephone  originally  exhibited  by  Prof.  Bell  consisted  of 
two  instruments  quite  similar  to  the  ear  pieces  or  receivers  which 
are  now  used.  One  of  these  instruments  was  used  as  a  receiver  and 
the  other  was  used  to  talk  into,  or  as  a  transmitter,  and  the  two  were 
connected  by  wires  (Fig.  99).  The  construction  of  these  instruments 
may  be  best  explained  by  reference  to  Fig.  100,  which  is  a  cut  of  a 
late  type  Bell  receiver.  In  this  figure,  R  represents  a  rubber  case, 

118 


UKE 


FIG,  99. 


FIG.  100. 


FIG.   101. 


NS,  a  magnet  tipped  with  a  piece  of  soft  iron,  and  W,  a  spool  of 
very  fine  wire  slipped  over  the  soft  iron  tip  of  the  magnet  and  con- 
nected to  the  binding  posts  at  the  end  of  the  rubber  case.  D  is  a 
diaphragm  or  disc  made  of  thin  varnished  iron.  This  diaphragm  is 
firmly  clamped  all  around  its  edges  in  such  a  position  that  its  center  is 
very  close  to  the  end  of  the  magnet  NS.  When  one  of  the  instru- 
ments is  brought  close  to  a  speaker's  mouth  the  waves  of  sound 
caused  by  his  speech  strike  the  diaphragm  and  cause  it  to  vibrate,  or 
move  back  and  forth.  The  speaking  end  of  the  instrument  is  formed 
into  a  mouthpiece  of  such  a  shape  that  it  gathers  in  a  large  volume  of 
the  waves  of  sound,  and  concentrates  their  effect  upon  the  diaphragm. 
Most  of  the  magnetic  lines  of  force  (Lesson  5,  page  34).  belonging 
to  the  magnet,  pass  through  the  coil  of  wire  and  many  of  them  enter 
the  iron  diaphragm  on  their  path  to  the  opposite  pole.  As  the  dia- 
phragm vibrates  from  the  effect  of  a  voice,  it  moves  back  and  forth  in 
front  of  the  magnet.  These  vibrations  are  too  small  to  be  seen  by 
the  eye,  but  they  are  of  sufficient  extent  to  cause  the  number  of  lines 
of  force  which  enter  the  disc  to  increase  considerably  as  it  approaches 
the  magnet,  and  decrease  as  it  moves  away  from  the  magnet.  In 


119 


this  way  the  distribution  of  the  lines  of  force  around  the  end  of  the 
magnet  is  altered  with  each  movement  of  the  disc,  and  the  number 
of  lines  of  force  which  pass  through  the  coil  of  wire  on  the  magnet 
is  increased  or  decreased  at  the  same  time.  It  is  an  experimentally 
determined  fact  that  when  a  change  occurs  in  the  number  of  lines  of 
force  passing  through  a  coil,  an  electric  pressure  is  set  up  in  the  coil. 
This  pressure  is  in  one  direction  when  the  number  of  lines  of  force 
passing  through  a  coil  is  increased,  and  in  the  opposite  direction 
when  the  number  is  decreased.  Consequently  the  movements  of  the 
Bell  telephone  diaphragm  set  up  electric  pressures  in  the  telephone 
coil,  and  when  this  coil  is  connected  by  wire  to  the  coil  of  another 
telephone,  waves  of  current  flow  through  the  circuit  which  corre- 
spond in  a  general  way  to  the  waves  of  sound  set  up  in  front  of  the 
diaphragm  of  the  first  telephone.  As  these  current  waves  flow 
through  the  coil  of  the  second  telephone,  they  increase  and  decrease 
the  strength  of  its  magnet.  This  alters  the  amount  of  the  attraction 
which  the  magnet  exerts  on  its  diaphragm,  and  the  diaphragm  is 
therefore  thrown  into  vibrations  which  correspond  with  the  current 
waves.  The  result  of  these  vibrations  of  the  second  diaphragm  is 
to  send  out  waves  of  sound  like  those  which  set  the  diaphragm  of  the 
first  telephone  to  vibrating. 

The  original  Bell  telephone  is  not  sufficiently  powerful  as  a  trans- 
mitter to  give  satisfactory  service,  but  it  is  an  extremely  sensitive 
and  satisfactory  receiver.  The  transmitters  which  are  now  generally 
used  are  therefore  based  on  an  entirely  different  principle. 

When  two  bits  of  carbon  are  permitted  to  lie  loosely  against 
each  other,  the  electrical  resistance  of  their  contact  is  very  much 
changed  when  changes  occur  in  the  pressure  of  the  contact ;  and  if  a 
blunt  metal  point  lie  loosely  against  a  carbon  surface,  differences 
of  pressure  of  the  contact  cause  variations  in  its  resistance. 
Advantage  is  taken  of  this  principle  in  the  common  telephone 
transmitter  ordinarily  known  as  the  Blake  Transmitter.  Fig.  101  is 
a  diagram  of  such  a  transmitter.  M  is  a  mouthpiece,  and  D  is  the 
diaphragm.  Touching  the  back  of  the  diaphragm  is  a  piece  of  plat- 
inum wire  about  ^  inch  in  diameter  and  ^  inch  long,  which  is 
soldered  into  a  hole  at  the  end  of  a  very  fine  german  silver  spring. 
The  other  end  of  this  piece  of  platinum  makes  a  loose  contact  at  C 
with  the  polished  face  of  a  carbon  button  which  is  suspended  on  a 
piece  of  very  flexible  watch  spring.  The  amount  of  pressure  at  the 
contact  is  exceedingly -small  and  may  be  very  delicately  adjusted  by 
the  screw  which  is  shown  near  the  bottom  of  the  figure. 

Platinum  and  carbon  electrodes  are  used  in  this  transmitter  in 
preterence  to  two  carbon  electrodes  because  there  is  less  sparking 
between  them  than  there  would  be  between  two  carbon  surfaces, 
and  the  conductivity  of  the  surfaces  is  preserved  for  a  longer  time. 
The  carbon  button  and  platinum  piece  are  represented  by  the  two 
heavy  black  spots  at  C  in  the  figure. 

120 


PUj^UMSA 


FIG.  102. 


FIG.  103. 


If  the  diaphragm  of  the  Blake  transmitter  is  spoken  to,  it 
vibrates  and  causes  the  platinum  point  to  press  more  or  less  lightly 
upon  the  carbon  button  and  thus  varies  the  resistance  of  the  contact. 
If  this  transmitter  be  connected  in  a  circuit  including  a  battery  and 
telephone  receiver,  as  shown  in  Fig.  101,  the  current  flowing  in  the 
circuit  will  vary  with  the  resistance  of  the  carbon  contact  when  the 
transmitter  diaphragm  vibrates.  The  current  in  the  circuit  is  there- 
fore thrown  into  waves  which  correspond  wim  the  vibrations  of  the 
diaphragm.  As  these  waves  of  current  pass  through  the  coil  of  the 
receiver  they  increase  and  decrease  the  strength  of  the  magnet  and 
its  diaphragm  is  thrown  into  vibration  so  that  the  original  sounds  are 
reproduced,  as  already  explained. 

Such  a  carbon  contact  as  is  used  in  a  transmitter  is  called  a 
microphone  and  a  transmitter  in  which  it  is  used  is  often  called  a 
microphone  transmitter.  A  very  easily  made  microphone  in  which 
both  electrodes  are  carbon,  is  shown  in  Figs.  102  and  103.  In  these 
figures,  C  represents  a  short  stick  of  carbon  with  pointed  ends, 
which  is  held  loosely  between  the  carbon  blocks  B  B.  These  blocks' 
are  a  little  countersunk  so  as  to  keep  the  carbon  stick  from  falling 
out  and  they  are  fastened  to  a  thin  piece  of  pine  board,  R.  The 
whole  is  mounted  on  a  board,  S.  When  this  microphone  is  con- 
nected in  circuit  with  a  cell  of  battery  and  a  telephone  receiver  by 
means  of  the  wires,  w  w,  which  are  attached  to  the  carbon  blocks, 
it  will  transmit  sounds  to  the  telephone.  Such  a  rough  microphone 
will  not  transmit  speech  so  that  it  can  be  understood,  but  it  will 
cause  the  telephone  to  sound  for  the  slightest  whisper. 

The  ordinary  microphone  transmitter  which  is  used  in  telephony 
is  a  rather  delicate  affair,  and  it  cannot  be  worked  with  more  than 
one  or  two  battery  cells.  In  order  that  long  lines  may  be  satisfac- 
torily spoken  over,  the  effect  of  the  transmitter  is  intensified  by  means 


121 


FIG.  104. 


FIG.  106. 


FIG.  105. 

of  electromagnetic  induction^  which  will  be  explained  in  a  later  les- 
son. The  induction  coil  (Fig.  104)  which  is  used  for  this  purpose  also 
has  the  advantage  of  shutting  out  a  scratchy  sound  which  is  caused 
by  the  transmitter. 

The  commercial  telephone  system  consists  of  much  more  than 
the  transmitter  and  receiver  with  their  accompanying  battery  and 
line.  When  telephones  are  used  simply  to  connect  two  points,  there 
must  be  located  at  each  point  a  transmitter,  a  receiver,  a  battery  cell, 
a  means  of  operating  an  electric  call  bell  at  the  other  point,  and  a 
local  call  bell.  This  outfit  is  usually  put  up  in  a  set  like  the  familiar 
form  shown  in  Fig.  105.  Here  A  represents  the  transmitter,  B,  the 
receiver,  C,  a  box  containing  the  battery  cell,  D,  the  electric  call  bell, 
and  B,  a  box  containing  a  small  dynamo  with  permanent  magnets 
called  a  magneto  which  may  be  operated  by  a  crank.  The  latter  is 
used  for  operating  the  call  bells.  When  the  receiver  is  not  in  use 
it  hangs  on  a  hook  (as  shown)  which  is  depressed  by  the  weight  of 
the  receiver  and  moves  electrical  contacts  which  connect  the  bells 
and  magneto  into  the  circuit  and  disconnect  the  telephone  instru- 
ments. When  the  receiver  is  taken  from  the  hook  the  latter  rises  so 


122 


FIG.  107. 

B,  call  bell.  M,  magneto  generator.  S,  hook  switch.  N,  O  and  P,  contact  springs, 
^receiver.  T,  transmitter.  C,  induction  coil.  D,  battery.  Land  L1,  line  terminals. 

that  the  contacts  cut  the  bells  and  magneto  out  of  circuit  and  the  tele- 
phone instruments  into  the  circuit.  Fig.  107  shows  a  diagram  of  the 
circuits  in  an  ordinary  commercial  telephone  set.  When  the  receiver 
is  hung  on  the  hook,  the  end  of  the  hook  lever  comes  in  contact  with 
P,  thus  connecting  the  magneto  generator  and  bell  into  the  line  cir- 
cuit which  connects  with  the  telephone  set  at  L  and  L1.  When  the 
receiver  is  removed  from  the  hook  a  spring  lifts  the  hook  and  brings 
the  lever  into  contact  with  N  and  O,  thus  connecting  the  secondary 
of  the  induction  coil  and  the  receiver  into  the  line  circuit  and  clos- 
ing the  battery  circuit  through  the  transmitter.  The  method  of 
using  the  telephone  is  too  well  known  to  require  description. 

The  use  of  telephones  simply  to  connect  two  points  is  only  a 
small  part  of  the  field  of  usefulness  of  the  telephone.  The  great  ma- 
jority of  telephones  are  used  in  connection  with  a  central  exchange. 
This  is  a  place  where  many  telephone  lines  center  and  are  brought 
to  a  switch  board  so  that  they  may  be  readily  connected  with  each 
other.  By  this  arrangement  each  telephone  user  in  a  great  city  may 
have  his  telephone  quickly  connected  with  that  of  any  other  person. 
Each  telephone  user  or  subscriber,  is  supplied  with  a  set  such  as  is 
shown  in  Fig.  105,  and  his  line  is  run  from  the  telephone  set  to  .a 


123 


section  on  the  switch  board  at  the  exchange  which  bears  the  sub\ 
scriber's  individual  number.  When  a  subscriber  wishes  to  speak 
with  another,  he  turns  the  crank  of  his  magneto,  thus  causing  a  signal 
at  the  switch  board.  He  then  takes  his  telephone  receiver  from  its 
hook,  and  when  the  switch  board  attendant  speaks,  he  asks  her  to 
connect  him  with  the  number  of  the  second  subscriber.  This  being 
done,  the  attendant  rings  the  telephone  bell  of  the  second  subscriber 
by  means  of  a  magneto,  which  calls  him  to  the  telephone.  When 
the  conversation  between  the  two  subscribers  is  completed,  one  of 
them  notifies  the  switch  board  attendant  by  means  of  his  magneto. 

The  earth  is  very  commonly  used  as  one-half  of  telephone  cir- 
cuits, so  that  only  one  wire  need  be  used  to  connect  an  instrument 
with  the  exchange.  The  ground  terminals  of  the  instruments  are 
then  connected  by  wire  to  gas  or  water  pipes,  or  to  iron  bars  driven 
into  the  ground.  The  telephone  receiver  is  such  an  exceedingly 
delicate  instrument,  that  outside  currents  are  likely  to  affect  its  oper- 
ation when  the  ground  return  is  used,  so  that  important  lines  are 
nowadays  constructed  with  a  complete  metallic  circuit,  that  is,  with 
wires  for  both  the  outgoing  and  incoming  part  of  the  line.  A  special 
transmitter,  called  the  long  distance  transmitter,  is  usually  used  with 
metallic  circuits.  This  transmitter,  which  is  shown  in  Fig.  106, 
is  a  microphone  transmitter  in  which  the  loose  contact  between  a  bit 
of  platinum  and  a  carbon  button,  is  replaced  by  a  short  tube  faced 
with  carbon  buttons  and  containing  powdered  carbon.  The  carbon 
granules  contained  in  the  tube  are  so  arranged  that  the  vibrations  of 
the  diaphragm  vary  the  pressure  with  which  they  lie  against  each 
other,  and  the  total  resistance  of  the  tube  passes  through  wide  varia- 
tions. This  transmitter  is  therefore  very  powerful,  and  is  especially 
used  on  lines  of  great  length. 

Copyrighted,  1894, 


124 


The  National  School  of  Electricity. 

REVIEW   OF   LESSON    XVI. 

Points  for  Review.     1.  How  does  a  Bell  telephone  work? 

2.  Why  is  the  Bell  telephone  not  commonly  used  as  a  transmitter? 

3.  What  is  a  microphone? 

4.  How  does  a  microphone  work? 

5.  How  does  a  Blake  transmitter  differ  from  an  ordinary  microphone? 

6.  What  apparatus  is  placed  in  telephone  sets? 

7.  What  is  the  purpose  of  each  part  of  the  set? 

8.  What  is  a  central  exchange? 

9.  What  is  the  purpose  of  a  telephone  switchboard? 

10.  What  is  meant  by  a  subscriber? 

11.  What  is  meant  by  a  ground  return?     What  is  meant  by  a  metallic  circuit? 

12.  How  does  the  essential  construction  of  a  long  distance  transmitter  differ  from 
that  of  an  ordinary  transmitter? 


XVII. 

THE    CONSTRUCTION    OF   TELEGRAPH    AND   TELE- 
PHONE  LINES   AND    INSTRUMENTS. 

An  overwhelming  proportion  of  the  telegraph  and  telephone 
lines  in  the  United  States  is  carried  on  the  wooden  poles  which  are 
such  common  sights  in  city  streets  and  along  railroads  and  high- 
ways. Usually  these  poles  are  made  of  white  cedar  or  chestnut, 
though  in  some  parts  of  the  country,  pine,  cypress  .  and  tamarack 
are  used.  The  poles  differ  in  length  from  twenty-five  feet  to  as 
much  as  one  hundred  feet,  and  in  diameter  at  the  upper  end  from 
four  or  five  inches  to  eight  or  ten  inches.  The  sizes  which  are  most 
commonly  used  are  from  twenty-five  to  forty  feet  long  and  from  five 
to  six  inches  in  diameter  at  the  top.  These  are  set  in  holes  dug  in 
the  ground  to  a  depth  varying  from  four  to  seven  feet,  depending 
upon  the  length  of  the  poles  and  the  importance  of  the  lines  which 
they  carry.  The  wires  which  are  supported  by  the  poles  are  usually 
tied  fast  to  glass  insulators,  which  are  screwed  on  oak  or  locust  pins 
(Fig.  1 08)  fastened  to  pine  cross-arms.  The  arms  are  usually  3^  x 
4J^  inches  and  as  long  as  required.  Fig.  109  shows  the  general 
arrangement  of  a  line  of  eighteen  wires.  The  cross-arms  fit  into 
notches  called  gains,  which  are  cut  in  the  sides  of  the  poles,  and 
they  are  then  fastened  in  place  by  means  of  lag  screws  (Fig.  no)  or 
bolts.  Poles  are  usually  erected  with  the  cross-arms  facing  each 
other  on  alternate  pairs  of  poles  (Fig.  in).  If  they  are  set  in  this 
manner,  the  arms  are  not  likely  to  be  pulled  off;  but  when  all  the 

125 


FIG.  108. 


FIG.  109. 


RE  I55UFJVICH.  I87g. 


FIG.  110. 


«-  if  -X.-I2T— • «— l2:-*«-l£:%-  15  • -£ 

ft  3  g  §  g  ,_! 


10- 


8     3     ft     ft 


8     8     8 


-ft     9     ft     8    8 


e   a 


9     ft     ft     ft    ft 


9     8    ft     8    ft 


FIG.  ill. 


FIG.  113. 


FIG.  112.  FIG.  114. 

arms  face  in  one  direction,  it  is  possible  for  all  of  them  to  be  pulled 
off,  one  after  another,  on  account  of  the  breaking  of  a  pole  or  of  one 
»f  the  arms.  Fig.  112  shows  the  top  of  a  pole  arranged  to  carry 
fifty  long  distance  telephone  wires.  The  numbers  marked  on  the 


FIG.  115. 

figure  show  the  dimensions.  In  this  figure  the  cross-arms  are  shown 
braced  with  iron  braces.  These  are  ordinarily  used  when  the  arms 
are  intended  to  carry  heavy  or  particularly  important  wires.  The 
braces  are  quite  commonly  used  in  cities,  but  are  not  often  used  in 
the  country,  except  upon  long  distance  telephone  lines. 

In  erecting  the  poles,  care  must  be  taken  that  they  be  set  in 
straight  lines  as  much  as  possible.  If  curves  or  corners  are  turned, 
the  poles  at  the  turn  must  be  braced  or  guyed  to  prevent  their  being 
pulled  over  by  the  strain  of  the  wires.  Figs.  113  and  114  show  a 
braced  pole  and  poles  guyed  in  two  ways.  When  the  guy  crosses 
a  street  or  other  passageway  it  is  not  uncommon  to  make  the  post 
to  which  the  guy  is  attached  eight  or  ten  feet  high.  The  figures 
show  the  poles  somewhat  tipped  or  inclined.  This  is  an  additional 
safeguard  against  the  poles  being  pulled  over  by  the  strain  of  the 
wires.  In  cities  the  poles  are  ordinarily  shaved  all  over  with  a  draw 
knife  before  being  set,  and  they  are  then  painted,  but  in  the  country 
this  refinement  is  not  necessary.  The  distance  between  poles  varies 
from  about  120  feet  to  300  feet  or  the  number  of  poles,  to  the  mile 
varies  from  45  to  18.  Only  in  the  case  of  important  lines  carrying 
many  wires  are  the  shorter  distances  between  poles  used,  and  the 
average  number  of  poles  used  for  lines  running  through  the  country 
is  from  20  to  30  per  mile. 

In  stringing  wire  various  methods  are  used,  but  the  commonest 
way  is  to  string  the  wire  out  on  the  ground,  after  which  it  is  carried 
to  its  place  on  the  insulators  by  linemen  who  climb  the  poles  by 
means  of  spurs,  or  else  the  wire  is  draw  from  a  reel  over  the  cross- 
arms.  Wire  which  is  intended  for  use  on  pole  lines  is  usually  fur- 
nished in  coils  which  may  be  laid  on  a  hand  reel  (Fig.  115).  When 
a  considerable  length  of  wire  has  been  laid  on  the  cross  arms,  a  block 
and  tackle  is  attached  to  its  end  and  it  is  stretched  up  tight.  While 
the  wire  is  held  tight,  it  is  tied  by  linemen  to  the  insulator  upon 
which  it  is  placed  at  each  cross-arm.  The  tie  is  usually  made  of 
wire  like  that  in  the  lines  but  it  is  often  somewhat  smaller  in  size. 
The  line  wire  is  laid  in  the  groove  of  an  insulator  and  one  end  of 
the  tie  wire  is  twisted  tightly  around  it  close  to  the  insulator.  The 
tie  wire  is  then  carried  around  the  groove  in  the  insulator  and  its 


FIG.  "116. 


FIG,  117. 


FIG.  118. 


FIG.  no. 

other  end  is  twisted  tightly  around  the  line  wire  close  to  the 
insulator.  The  appearance  of  the  line  wire  and  loop  of  tie  wire  with 
the  insulator  removed,  is  shown  in  Fig.  116. 

Wire  is  furnished  from  the  wire  mills  in  coils  which  contain 
lengths  varying  from  a  few  hundred  feet  to  a  half  mile  or  more.  A 
great  many  joints  are,  therefore,  necessary  in  a  long  line.  The  com- 
monest form  of  joint  used  in  this  country  is  that  known  as  the  Wes- 
tern Union  or  twist  joint,  which  is  shown  in  Fig.  117.  The  figure 
shows  the  way  a  twist  joint  appears  when  made  of  iron  wire  of 
medium  size  and  also  when  made  of  small  copper  wire.  In  impor- 
tant lines,  these  joints  are  always  soldered  in  order  that  their  electri- 
cal conductivity  may  be  as  great  as  possible,  and  that  the  wires  at 
the  joint  may  not  be  corroded  by  the  effect  of  gases  which  are  in  the 
air.  The  soldering  is  done  by  dipping  the  twisted  joint  into  a  pot  of 
melted  solder.  Another  form  of  joint  which  is  often  used  with  cop- 
per wire  is  that  known  as  the  Mclntyre  sleeve  joint.  For  this  pur- 
pose a  '4  Mclntyre  sleeve  n  made  of  two  short  pieces  of  copper  tube 
brazed  together  side  by  side  (Fig.  118)  is  furnished.  The  tubes  are 
of  such  size  that  the  line  wire  slips  easily  into  them.  The  ends  of 
the  wire  are  slipped  through  the  tubes  from  opposite  directions,  and 
are  then  turned  up  and  cut  off  close.  The  joint  is  then  twisted  by 
special  pliers  as  shown  in  Fig.  119.  The  twisting  draws  the  tubes 
so  closely  around  the  wire  as  to  insure  a  good  electrical  contact  and 


128 


FIG.  120. 

no  soldering  is  required.  The  latter  point  is  of  particular  advantage 
when  the  line  is  composed  of  what  is  known  as  hard  drawn  copper 
wire,  as  heat  due  to  soldering  spoils  the  temper  of  such  wire  and 
reduces  its  strength. 

The  insulation  of  long  telegraph  and  telephone  lines  is  a  matter 
of  much  importance.  The  effect  of  poor  insulation  is  illustrated  in 
Fig.  1 20,  where  A  and  B  are  two  telegraph  stations  connected  by  a 
wire  strung  upon  poles.  The  electrical  circuit  through  the  stations 
is  completed  by  a  ground  return.  All  the  battery  is  supposed  to  be 
located  at  A.  If  the  wire  is  not  well  insulated  at  each  point  where 
it  is  supported  at  the  poles,  an  appreciable  proportion  of  the  cur- 
rent leaks  out  of  the  line  into  the  earth  without  having  passed 
through  the  distant  station.  Each  point  of  leakage  gives  a  branch 
circuit,  and  if  the  line  is  long,  so  that  many  such  branch  circuits  are 
in  parallel,  the  total  leakage  may  be  so  great  that  sufficient  current 
does  not  reach  station  B  to  work  its  relay  when  signals  are  made  at 
A  by  means  of  the  key.  The  actual  proportion  of  the  current  which 
escapes  by  leakage  depends  upon  the  ratio  of  the  line  resistance  to 
the  combined  resistance  of  all  the  leakage  paths. 

The  effect  of  leakage  may  even  be  so  great  as  to  permit  enough 
current  to  flow  through  the  relay  at  A  to  keep  its  armature  attracted 
when  the  key  is  open  at  B.  This  makes  it  impossible  to  send'  sig- 
nals from  B  to  A.  To  avoid  the  reduction  of  current  in  the  relay  at 
one  end  of  a  line,  which  may  be  caused  by  leakage  when  all  the  battery 
is  placed  at  the  other  end,  it  is  usual  to  divide  the  battery  and 
put  one-half  of  it  at  each  end  of  the  line,  care  being  taken  that  the 
two  halves  are  properly  connected  in  series.  In  this  case  the  full 
amount  of  current  will  always  flow  through  both  relays  when  the 
keys  are  closed,  but  the  leakage  may  be  so  great  that  the  relay 
armature  at  either  station  is  continuously  attracted  without  regard 
to  whether  the  circuit  is  made  or  broken  by  the  key  at  the  other 
station,  in  which  case  signals  cannot  be  transmitted  from  one  station 
to  the  other. 

To  make  the  resistance  of  the  leakage  paths  as  great  as  possible, 
the  insulators  to  which  the  line  wire  is  attached  are  commonly  made 


129 


J    S 


FIG.  121. 


FIG.  125.  FIG.  122.  FIG.  124. 

of  glass  in  the  form  of  bells  (Fig.  121)  which  may  be  screwed  on 
the  wooden  pins  in  the  cross  arms.  These  insulators  are  made  in 
various  slightly  different  forms  and  sizes,  as  shown  in  the  figure. 
Glass  is  an  excellent  insulator  when  it  is  dry  (L,esson  i,  page  4)  but 
in  damp  weather  its  surface  becomes  covered  with  a  thin  layer  or 
film  of  water.  This  film  of  water  makes  a  path  through  which,  at 
every  insulator,  a  small  portion  of  current  may  leak  from  the  wire  to 
the  wooden  supporting  pin,  and  thence  over  the  damp  wood  of  the 
cross  arm  and  pole  to  the  ground,  or  to  some  other  wire,  Water  is 
a  comparatively  poor  conductor  and  the  quantity  of  current  which 
escapes  at  each  insulator  is  very  small,  but  the  total  loss  at  all  the 
insulators,  on  a  line  several  hundred  miles  long,  may  be  a  very 
serious  matter.  As  the  leakage  at  each  insulator  is  along  the 
film  of  water  which  covers  the  surface  of  the  glass,  the  effective  insu- 
lation is  increased  by  increasing  the  length  of  the  path  over  which 
the  current  must  pass  on  the  insulator's  surface.  This  is  done  by 
adding  a  second  petticoat  or  bell  to  the  glass  as  shown  in  Fig.  122. 
Rubber  hook  insulators  (Fig.  123)  which  may  be  screwed  into 
the  bottoms  of  cross  arms  or  in  other  similar  positions  are  sometimes 


130 


used  for  special  work.  In  Europe,  porcelain  bell  insulators  which 
are  quite  similar  in  shape  to  the  American  glass  insulators,  are  used. 
The  dense  white  porcelain  of  which  these  insulators  are  made  is 
better  for  the  purpose  than  is  glass,  but  it  is  more  expensive. 

Where  telegraph  or  telephone  lines  run  inside  of  buildings,  it  is 
usual  to  use  copper  wire  of  a  small  size  which  is  insulated  by  a 
braided  covering  of  cotton  thread  thoroughly  soaked  with  paraffine. 
Where  single  lines  need  support,  as  in  passing  from  the  poles  to  a 
building,  it  is  usual  to  use  an  oak  bracket  (Fig.  124}  with  glass  insulator 
or  a  porcelain  knob  (Fig.  125)  fastened  at  some  convenient  point 

It  has  already  been  said  that  in  telegraphy  and  telephony  the 
earth  is  commonly  used  as  one-half  of  the  electric  circuit  (Lesson  XIV, 
page  104,  and  Lesson  XVI,  page  131).  The  connection  to' the  earth  in 
telegraphy  is  made  by  means  of  ground plates,  which  may  be  sheets  of 
copper  or  galvanized  iron  TV  inch  thick  and  three  or  four  feet  square. 
The  plates  are  buried  in  the  earth  in  a  vertical  position  where  the 
soil  is  damp.  Very  often  a  connection  to  gas  and  water  pipes  may 
be  substituted  for  the  ground  plate.  In  this  case,  the  pipe  is  care- 
fully cleaned  at  the  point  to  which  the  conductor  is  to  be  attached 
and  the  wire  is  soldered  fast  to  it.  For  telephony  a  sufficiently  sat- 
isfactory ground  may  usually  be  made  for  individual  lines  by  driving 
an  iron  rod  several  feet  into  the  ground,  though  a  connection  to 
water  or  gas  pipes  is  better.  At  central  exchanges  a  ground  plate  is 
almost  always  used. 

In  large  cities  telephone  and  telegraph  conductors  are  often 
placed  underground,  and  in  some  places  they  are  run  over  house  tops. 
When  electric  wires  are  placed  underground  they  must  be  continu- 
ously insulated  by  some  material  which  is  sufficiently  flexible  to  per- 
mit of  the  wires  being  easily  handled.  For  telegraph  wires  this 
insulation  commonly  consists  of  a  thickness  of  a  vulcanized  rubber 
compound  which  has  been  placed  on  the  wire  under  hydraulic  pres- 
sure. In  the  case  of  telephone  wires.it  is  particularly  important  that 
their  capacity  be  the  smallest  that  is  possible,  on  account  of  the 
delicacy  of  the  telephone  current,  and  crinkled  paper  is  often  used 
for  their  insulation  (refer  to  Lesson  XII,  page  88).  When  a  fibrous 
insulation  such  as  paper  is  used,  it  is  necessary  to  protect  it  from 
absorbing  moisture,  and  a  lead  covering  over  the  insulation  is  there- 
fore used.  In  fact,  the  lead  covering  is  generally  used  with  rubber 
covered  wires  also,  in  order  that  the  rubber  may  be  properly  pro- 
tected from  mechanical  injury  and  from  the  injurious  action  on  its 
insulating  qualities  of  gases  or  liquids  which  may  come  in  contact 
with  it  when  under  ground. 

Before  the  lead  covering  is  put  on  the  insulated  wires,  a  number 
of  them  are  usually  '  'laid  up' '  or  bunched  into  a  cable,  and  the  lead 
is  put  around  the  whole.  This  may  be  done  by  pulling  the  cable 
into  a  lead  pipe,  or  by  making  a  pipe  around  the  cable  by  means  of 


131 


FIG.  126. 


FIG.  127. 


FIG.  128. 
a  lead  press.     The  end  of  a  telephone  cable  is  shown  in  Fig.  126. 

Underground  cables  are  usually  not  buried  directly  in  the 
ground,  but  are  placed  in  what  are  known  as  electric  conduits. 
These  consist  of  pipes  or  ducts  made  of  iron,  terra  cotta,  cement, 
wood  and  sometimes  other  materials.  The  ducts  are  sometimes  laid 
singly  but.  they  are  usually  laid  in  sets  surrounded  by  concrete,  as 
shown  in  Fig.  127,  which  is  an  end  view  of  a  conduit  containing 
twelve  ducts.  The  ducts  are  commonly  circular  in  cross  section  and 
from  two  to  three  inches  in  diameter,  though  ducts  of  rectangular 
cross  sections  are  often  used. 

In  order  that  cables  may  be  placed  in  the  conduits,  arrange- 
ments for  getting  at  the  ducts  must  be  made.  This  is  done  by  build- 
ing cable  manholes  at  intervals  along  the  conduit.  These  are  usually 
brick  vaults,  six  or  seven  feet  deep  and  several  feet  in  diameter, 
which  are  covered  at  the  street  surface  by  cast  iron  covers.  Sections 
of  the  conduit  terminate  on  opposite  sides  of  the  manholes  as  shown 
in  Fig.  128.  The  manholes  are  placed  at  intervals  of  about  300  feet 
in  straight  parts  of  a  conduit  and  also  at  the  turns  which  may  occur 
in  the  conduit. 

When  a  conduit  with  its  manholes  is  all  built,  the  cables  are 
drawn  into  the  ducts,  one  section  at  a  time.  The  sections  of  each 
cable  must  be  jointed  together  in  the  manholes.  To  do  this  the 
conductors  are  first  jointed  in  the  usual  manner  and  their  joints  are 
insulated.  Finally  a  short  piece  of  lead  pipe  is  placed  over  the  joints 
and  is  soldered  at  both  ends  by  a  wiped  joint  to  the  lead  coverings. 
This  makes  the  joint  moisture-proof  if  it  has  been  properly  made. 
Making  cable  joints  requires  the  greatest  care  to  avoid  the  entrance  of 
moisture  into  the  cable,  and  it  is  always  necessary  to  handle  open 
cable  ends  with  the  greatest  care.  The  ends  should  always  remain 
sealed  except  when  work  is  to  be  done  on  them. 


132 


FIG   131. 


FIG.  129. 


FIG.  130. 


In  cities  where  the  telephone  wires  are  underground  it  is  usual 
to  distribute  the  lines  to  the  subscribers  from  the  manholes.  The 
wires  for  all  the  subscribers  in  a  block  may  be  taken  in  a  cable  from 
a  manhole  to  the  basement  of  a  building,  and  they  may  then  be  dis- 
tributed to  the  subscribers  by  passing  through  or  over  the  buildings. 
Where  cables  terminate  in  buildings,  or  on  poles  at  points  where  the 
underground  wires  connect  to  overhead  lines,  it  is  usual  to  carry  each 
cable  into  a  small  water-tight  box,  called  a  cable  head  or  terminal. 
Connections  pass  through  the  sides  of  the  box  so  that  the  overhead 
wires  may  be  connected  to  those  in  the  cable.  Fig.  129  shows  a  tel- 
ephone pole  with  a  large  box  arranged  to  contain  a  cable  head.  Fig. 
130  shows  a  terminal  head  designed  to  terminate  a  cable  containing 
one  hundred  wires.  When  overhead  and  underground  wires  are 
joined  they  are  always  connected  to  a  lightning  arrester,  which 
usually  consists  of  two  brass  plates  with  saw  tooth  edges  like  Fig. 
131.  One  of  the  plates  is  connected  to  the  wire  and  the  other  plate 
to  the  ground.  If  the  wire  becomes  charged  by  lightning  a  dis- 
charge jumps  from  the  wire  to  the  ground  across  the  space  between 
the  teeth,  and  thus  relieves  the  wire. 

Where  cables  terminate  at  city  telegraph  offices  or  telephone 
exchanges  the  wires  are  carried  from  the  terminal  heads  to  distribu- 
tion boards  and  from  there  to  the  switchboard.  When  overhead 
wires  enter  large  offices  or  exchanges  they  are  also  carried  through 
a  distribution  board  to  the  switchboard. 

The  object  of  the  distribution  board  is  to  enable  any  connection 
between  the  line  wires  and  the  switchboard  to  be  made  without 
changing  the  permanent  connections  either  at  the  switchboard  or 
the  cable  terminals.  The  wires  are  connected  to  lightning  arresters 
at  the  distribution  board  and  also  to  very  delicate  fuses.  The  latter 
are  made  of  bits  of  gold  leaf  or  very  fine  wire,  and  are  used  to  pro- 


133 


FIG.  132. 


FIG.  133. 


FIG.  134. 


FIG.  135 


tect  the  switchboard  apparatus  and  office  instruments  from  being 
burned  out  by  too  great  a  current.  A  burn  out  would  be  the  certain 
result  of  a  telephone  or  telegraph  wire  making  contact  (crossing  it  is 
called)  with  a  live  electric  light  wire,  were  it  not  for  the  fuse,  which 
melts  and  breaks  the  circuit  when  too  large  a  current  flows,  and  thus 
saves  the  intruments. 

Telegraph  switchboards  are  quite  simple  in  construction  except  in 
the  largest  terminal  stations.  For  small  way  stations  they  consist 
simply  of  a  spring  jack  (Fig.  132)  and  a  divided  brass  plug  attached 
to  a  double  conducting  cord  (Fig.  133).  The  line  enters  and  leaves 
the  switch  through  the  binding  posts,  one  of  which  is  connected  to 
the  spring  seen  in  the  figure  and  the  other  of  which  is  connected 
to  a  contact  against  which  the  spring  presses.  When  the  divided 
plug  is  inserted  between  the  spring  and  the  fixed  contact,  the  cur- 
rent is  forced  to  flow  through  the  conductors  attached  to  the  plug  and 
through  the  station  instruments.  For  stations  which  are  entered  by  a 
number  of  lines,  a  spring  jack  switchboard  is  arranged  as  shown  in 
Fig-  134.  A  plain  plug  board  is  more  common  for  way  stations  at 
which  a  number  of  lines  enter.  The  appearance  of  this  is  shown  in 
Fig.  135.  The  board  shown  in  the  figure  is  provided  with  four 
vertical  brass  bars  to  which  the  line  wires  are  connected.  Between 
the  bars  are  rows  of  brass  discs.  The  discs  and  the  binding  post 
which  are  on  a  horizontal  line  are  connected  together  by  copper 
wires  at  the  back  of  the  board,  and  to  these  binding  posts  the  station 
instruments  are  connected.  By  properly  connecting  the  discs  and 


134 


J[   J^ 

?*w  T*v 

o 


G  O 


—3!  — 

E  W   E  \V 

o  o  o  o 


n 

(%-j 

0- 

£ . . . 

FIG.  136.  FIG.  137. 

bars  together  by  inserting  plugs  in  the  holes  shown  in  the  figure, 
the  station  instruments  may  be  connected  into  the  circuits  or  the 
station  may  be  entirely  cut  out  of  service,  at  the  will  of  the  operator. 
Fig.  136  shows  more  plainly  the  effect  of  plugging  the  circuits.  In 
this  the  vertical  lines  represent  the  bars  to  which  the  line  wires 
are  connected  and  the  horizontal  lines  represent  the  rows  of  discs  to 
which  the  instruments  are  connected.  In  the  left  hand  cut  of  the 
figure,  instruments  A  are  connected  to  line  number  one,  and  instru- 
ments B  to  line  number  two;  in  the  right  hand  cut  the  A  instruments 
are  connected  to  line  number  two,  and  the  B  instruments  are  cut  out 
without  interrupting  line  number  one,  since  the  current  can  flow  in 
upon  one  bar  to  a  plug,  then  across  the  disc  to  another  plug  and  out 
again  on  the  other  bar.  The  upper  horizontal  line  marked  G  in  the 
figure,  is  used  in  grounding  the  circuits  when  desired.  In  large  ter- 
minal stations  a  combination  of  the  spring  jack  and  plug  switch- 
boards are  made  so  that  the  lines,  batteries,  and  instruments  may  be 
connected  together  as  desired. 

Telephone  switchboards  are,  as  a  rule,  more  complicated  than 
telegraph  boards,  since  an  exchange  is  always  connected  to  a 
larger  number  of  wires  than  is  a  telegraph  office  of  equal  importance, 
and  since  the  connections  of  the  telephone  wires  must  be  arranged 
so  that  the  operator  and  subscribers  are  able  to  signal  and  talk  to 
each  other,  as  well  as  so  that  the  subscribers'  lines  may  be  quickly 
connected  together. 

In  the  earlier  and  simpler  forms  of  telephone  switchboards  the  sub- 
scribers' wires  on  entering  the  exchange  are  each  connected  to  a  switch- 
board circuit  which  contains  a  spring  jack  and  an  electromagnet,  which 
controls  a  drop  shutter,  and  which  terminates  at  the  ground  plate. 
One  form  of  this  electromagnet  is  seen  in  Fig.  137.  The  armature, 
A,  of  the  electromagnet,  M,  has  a  hook,  D,  which  ordinarily  sup- 
ports the  shutter  or  drop,  P,  which  is  hinged  at  the  bottom,  but 
when  the  subscriber  sends  current  over  the  line  from  his  magneto  the 
armature  is  attracted  and  the  drop  is  released.  When  released,  the 
drop  falls  on  its  hinge  and  shows  the  subscriber's  number,  which  is 
painted  at  its  back.  When  a  subscriber  calls,  the  exchange 
operator  inserts  a  plug  in  the  subscriber's  spring  jack,  which  connects 
her  telephone  instruments  to  his  line.  She  then  learns  what  connec- 
tion is  desired,  and  makes  it  by  inserting  the  plugs  at  the  ends  of  ,a 


135 


0! 

XTNIVER3I 
jRNiA; 


SUBSCRIBERS 

•• 


SECTION  N 


SECTION  N?3- 


SECf  ION  N?  2. 
EXCHANGE 

FIG.    139. 

cord  into  the  spring  jacks  belonging 
to  the  lines  of  the  two  subscribers  that 
are  to  be  connected  together.  When 
the  subscribers  have  finished  talking, 
one  of  them  turns  his  magneto  crank, 
which  causes  a  drop  to  fall  in  the  ex- 
change and  calls  the  operator's  atten- 
tion to  the  fact  that  the  lines  may  be 
disconnected.  Boards  of  this  general 
type  are  used  in  nearly  all  small  ex- 
changes in  the  country.  One  is  shown 
in  Fig.  1 38.  It  will  be  noticed  that 
the  cords  are  held  down  under  the 
table  by  weights  running  on  pulleys. 
This  keeps  the  cords  from  getting  tan- 
gled, and  the  plugs  are  held  in  a  con- 
venient position  for  the  operator  to  pick  up. 

As  one  operator  can  take  care  of  the  calls  from  only  a  limited  num- 
ber of  subscribers  (50  to  100  subscribers  is  the  usual  number  per 
operator),  in  the  larger  exchanges  a  great  many  boards  of  the  kind 
described  would  be  required,  and  much  difficulty  and  waste  of  time 
would  be  experienced  in  making  connections  between  the  line  of 
a  subscriber  connected  to  one  board  and  the  line  of  a  subscriber  con- 
nected to  another  board  in  another  part  of  the  room.  Hence,  what 
are  known  as  multiple  switchboards  are  used  in  exchanges  having 


FIG.  138. 


136 


many  subscribers.  The  multiple  board  with  its  numerous  details  can 
be  explained  here  only  in  the  briefest  outline.  The  principle  upon 
which  it  is  based  is  to  divide  the  total  number  of  subscribers'  lines 
into  sets,  each  of  which  is  brought  to  a  different  sectipn  of  the  switch- 
board where  the  lines  belonging  to  the  set  may  be  looked  after  by 
an  operator.  The  lines  are  connected  to  a  drop  and  a  spring  jack  in 
their  proper  sections,  so  that  the  operator  may  communicate  with 
the  subscribers  by  means  of  her  telephone  set.  In  addition  to  enter- 
ing its  own  section  through  a  drop  and  spring  jack,  every  subscriber's 
line  is  also  connected  to  a  spring  jack  in  every  other  section.  Con- 
sequently each  operator  attends  to  the  calls  of  a  limited  number  of 
subscribers  whose  lines  are  connected  to  drops  in  her  section,  and 
since  all  other  lines  have  spring  jacks  in  her  section  she  can  connect 
any  of  her  subscribers'  lines  to  the  line  of  any  other  subscriber 
which  enters  the  exchange.  Fig.  139  shows  the  principle  of  the 
multiple  board.  The  dots  marked  "local  jacks"  in  each  section 
represent  the  spring  jacks  belonging  to  the  lines  which  are  looked 
after  by  the  operator  at  the  section^  For  simplicity  the  drops,  keys 
for  ringing  up  subscribers,  operator's  telephone  set,  etc.,  are  omitted 
from  the  figure.  The  dots  marked  "ordinary  jacks' '  represent  the  mul- 
tiple spring  jacks,  by  means  of  which  the  operator  may  connect  her 
subscribers  with  any  others.  It  will  be  seen,  for  instance,  that  sub- 
scribers' lines,  numbers  i,  2,  and  3,  enter  the  local  jacks  of  section 
number  one,  but  they  also  enter  the  ordinary  jacks  of  the  other  sec- 
tions. If  an  operator  in  the  second  section  wishes  to  connect  one  of 
her  wires,  say  number  6,  with  one  of  those  belonging  to  the  first  sec- 
tion, say  number  3,  she  is  able  to  do  so  at  once  on  her  own  part  of 
the  board,  as  shown  in  the  figure.  An  ingenious  arrangement  by 
which  the  operator  can  tell  when  a  line  is  in  use,  prevents  switching 
three  subscribers  together.  Exchanges  with  multiple  switchboards 
have  been  planned  to  give  telephone  service  to  the  enormous  number 
of  10,000  subscribers  from  one  board,  but  the  telephone  service  has 
never  yet  risen  in  any  place  to  such  magnitude  that  any  exchange 
has  reached  the  number  of  10,000  subscribers.  In  the  large  cities  it 
is  usual  for  the  purpose  of  economizing  in  the  construction  of  lines  to 
locate  several  sub-exchanges  to  serve  districts  at  a  considerable  dis- 
tance from  the  main  exchange.  This  practice  causes  the  number  of 
subscribers'  lines  attached  to  any  one  exchange  to  be  smaller  than 
might  be  otherwise  expected.  , 

Copyrighted,  1894, 


137 


The  National  School  of  Electricity. 

REVIEW  OF  LESSON  XVII. 

Points  for  review.     1.    What  wood  is  usually  used  for  telegraph  and  telephone 
poles?     For  cross  arms?     For  pins? 

2.  Why  are  corner  poles  braced  or  guyed? 

3.  How  are  joints  in  line  wire  made? 

4.  Why  is  good  insulation  so  important  for  telegraph  lines? 

5.  Of  what  material  and  in  what  form  are  the  insulators  which  are  used  on  pole 
lines  usually  made? 

6.  Why  is  the  insulation  of  telegraph  and  telephone  lines  likely  to  be  poor  in  damp 
or  wet  weather? 

7.  Why  is  insulated  wire  used  for  telegraph  and  telephone  lines  where  they  arc 
inside  of  buildings? 

8.  What  are  ground  plates? 

9.  What  materials  are  used  for  insulating  underground  telegraph  and  telephone 
wires? 

10.  Why  are  lead  coverings  put  on  underground  cables? 

11.  What  are  electric  conduits?     What  are  manholes? 

12.  How  are  cables  terminated  in  buildings? 

13.  What  is  the  purpose  of  fuses  in  telegraph  and  telephone  circuits? 

14.  How  are  the  common  forms  of  telegraph  switchboards  arranged? 

15.  How  are  the  simpler  forms  of  telephone  switchboards  arranged? 

16.  What  is  the  principle  of  multiple  telephone  switchboards? 

17.  Why  are  multiple  switchboards  necessary  in  large  exchanges? 


WESSON   XVIII. 

TESTING  LINES  FOR  INSULATION  AND  CONDUCTIVITY 
AND  THE  LOCATION  OF  LEAKS  AND  BREAKS. 

On  account  of  their  exposed  positions,  overhead  telegraph  and 
telephone  wires  are  particularly  liable  to  injury.  It  is  therefore 
necessary  to  make  careful,  systematic  and  continued  tests  of  import- 
tant  lines  in  order  that  they  may  be  kept  in  satisfactory  condition. 
The  troubles  to  which  lines  are  heir  may  be  divided  into  four  classes: 

1.  Grounds; 

2.  Crosses; 

3.  Poor  connections; 

4.  Breaks. 

A  line  is  said  to  be  grounded  when  so  much  current  leaks  from 
it  to  the  ground  as  to  interfere  with  its  proper  use.  Grounding  may 
be  caused  by  a  general  leakage  all  along  the  line,  or  a  large  leak 
may  exist  at  one  point,  where  the  line  comes  in  contact  with  trees, 
etc.  When  the  insulation  of  the  line  becomes  so  low  that  practically 
all  the  current  leaks  off,  it  is  said  to  be  dead  grounded. 

13& 


Lines  are  said  to  be  crossed  when  they  make  contact  with  each 
other,  so  that  messages  sent  over  one  line  may  be  received  on  the 
other.  Only  one  of  the  crossed  lines  can  then  be  used  to  send  inde- 
pendent messages.  The  most  fruitful  cause  of  crosses  is  the  swinging 
of  loose  wires  in  the  wind,  by  which  means  they  become  tangled  up. 
Sometimes  crosses  or  grounds  will  appear  and  disappear  at  intervals, 
when  they  are  often  called  swinging  crosses  or  grounds.  These  may 
be  caused  by  a  swinging  wire  which  touches  another  wire  or  a 
ground  contact  at  intervals,  but  does  not  remain  continuously  in 
contact. 

Poor  connections  result  from  various  causes,  such  as  corroded 
joints  in  the  wire,  a  corroded  connection  to  a  ground  plate  or  water- 
pipe,  a  poor  contact  between  the  ground  plate  and  the  earth,  loose 
connections  at  binding  posts  of  instruments,  at  switchboards  or  at 
batteries.  Poor  connections  may  very  seriously  reduce  the  con- 
ductivity of  the  line,  and  thus  reduce  the  distinctness  of  messages 
sent  over  it  unless  extra  battery  power  is  used. 

A  break  may  be  caused  by  a  binding  post  connection  working 
entirely  loose,  by  a  wire  breaking  at  an  instrument,  or  by  the  line  wire 
breaking.  It  may  also  be  caused  by  defective  contacts  in  the  work- 
ing parts  of  an  instrument,  or  in  the  case  of  a  telegraph  line  by  a 
careless  operator  leaving  his  key  open.  When  a  line  wire  breaks, 
the  circuit  may  be  entirely  opened,  or  if  one  or  both  of  the  ends  get 
on  the  ground  it  may  be  possible  to  get  current  through  one  or  both 
portions. 

The  simplest  way  of  determining  the  condition  of  a  line  is  by 
comparing  the  distinctness  of  the  signals  which  are  transmitted  over 
it  from  day  to  day.  Thus,  in  the  case  of  a  telegraph  line,  if  the  sig- 
nals which  are  sent  out  from  a  terminal  station  where  half  the  bat- 
tery is  located,  are  found  to  be  strong  and  good  on  a  certain  day, 
while  signals  which  are  received  at  the  station  over  the  same  wire 
are  weak  and  indistinct,  it  is  evident  that  the  insulation  of  the  line 
is  poor  (compare  Lesson  XVII,  page  129).  If  the  signals  which  are  sent 
and  received  are  equally  indistinct,  while  the  battery  is  in  good  con- 
dition, the  conductivity  of  the  line  is  probably  less  than  usual.  If 
signals  sent  over  one  wire  can  be  received  on  another,  the  lines  are 
either  crossed  or  sufficient  current  leaks  from  one  wire  to  the  other  to 
give  the  effect  of  a  cross.  In  the  case  of  a  break  which  opens  the  cir- 
cuit, the  armatures  of  the  relays  in  the  line  fall  back  from  their  mag- 
nets; but  if  the  ends  of  the  line  at  the  break  become  grounded, it  may  be 
possible  to  send  signals  between  stations  upon  the  same  side  of  the 
break. 

The  section  between  two  stations  upon  which  trouble  exists,  may 
usually  be  located  in  the  case  of  a  local  telegraph  line  passing  through 
stations  which  are  close  together.  To  do  this,  the  station  nearest  one 
end  is  called  up  from  the  end  station,  either  by  means  of  the  faulty  wire 
or  by  means  of  another  wire,  and  is  told  to  ground  the  faulty  wire. 

139 


) 


FIG.  140. 

This  being  done,  signals  are  transmitted  between  the  two  stations 
over  the  faulty  wire.  If  the  wire  works  all  right,  the  next  station  is 
called  up  and  the  test  of  the  working  condition  of  the  wire  is  again 
made.  This  is  continued  from  station  to  station  until  the  signals 
fail  in  transmission.  The  trouble  is*  then  on  the  last  section  tested, 
and  a  line-man  may  be  sent  out  to  exactly  locate  and  correct  it. 

Trouble  on  telephone  lines  is  usually  shown  to  the  exchange 
operator  through  difficulty  or  impossibility  in  communicating  with  a 
subscriber.  Crosses  make  themselves  evident  through  the  fact  that 
when  a  subscriber  on  one  of  the  crossed  wires  calls  the  exchange  by 
working  his  magneto,  not  only  does  the  drop  fall  which  is  attached  to 
his  wire,  but  the  drops  attached  to  the  wires  which  are  crossed  with 
his  also  fall.  What  is  known  as  cross  talk  between  telephone  wires 
is  not  a  certain  sign  of  a  cross,  as  it  may  be  caused  by  either  electro- 
magnetic or  electrostatic  induction  (Lesson  I).  The  latter  is  a  very 
common  cause  of  cross  talk.  To  avoid  cross  talk  in  telephone  cables, 
the  two  wires  of  each  metallic  circuit  are  twisted  together,  and  such  a 
cable  is  therefore  said  to  be  made  up  of  twisted  pairs. 

In  the  case  of  long  trunk  telegraph  or  telephone  lines  con- 
necting cities  at  considerable  distance  apart,  methods  are 
required  for  the  location  of  faults  by  direct  electrical  measurements. 
It  is  usual  to  make  careful  daily  or  weekly  measurements  of  the 
insulation  and  conductivity  and  sometimes  of  the  capacity  of  such 
lines.  The  results  of  these  measurements  are  carefully  recorded  in  a 
book;  and  the  records  are  a  material  aid  in  the  location  of  faults  by 
electrical  measurement.  The  commonest  instruments  to  be  used  in 
testing  lines  are  a  sensitive  galvanometer  (Lesson  IX,  page  58)  and  a 
Wheatstone  bridge  (Lesson  X,  page  68) . 

To  measure  the  conductivity  of  a  metallic  circuit  is  very  sim- 
ple. At  one  end  of  the  line  the  two  wires  are  connected  together, 
and  at  the  other  end  of  the  line  the  two  wires  are  connected  to  the 
bridge  (Fig.  140).  Half  the  resistance  measured  by  the  bridge  is 
the  resistance  of  one  of  the  wires  composing  the  circuit,  if  the  wires 
are  of  equal  length  and  size. 

When  only  one  wire  is  available,  as  may  be  the  case  with  tele- 
graph circuits,  its  far  end  is  connected  to  ground,  the  near  end  is 
connected  to  one  of  the  binding  posts  of  the  bridge,  and  the  other 


14,0 


EARTH 


FIG.    141. 

binding  post  to  which  the  unknown  arm  should  be  connected,  is 
connected  to  ground  as  shown  at  B1,  in  Fig.  141,  which  shows  the 
arrangements  of  the  connections  for  a  postoffice  pattern  bridge  (lyes- 
son  X,  page  70).  With  this  arrangement  the  arm  of  the  bridge 
marked  A  in  the  diagrams,  Fig.  140,  and  Lesson  X,  Fig.  47,  is 
between  A  and  B.  The  arm  B  is  between  A  and  E,  in  the  zigzag 
part  of  the  bridge  rheostat.  The  arm  C  is  between  B  and  C ;  and  the 
unknown  resistance  D  is  connected  to  the  bridge  at  the  points  C 
and  E.  The  resistance  measured  by  the  bridge  may  be  taken  as 
equal  to  the  resistance  of  the  line,  provided  the  ground  connections 
are  good. 


KVo  JL 


FIG.  142. 

When  the  individual  conductivity  of  three  wires  running 
between  the  same  points  is  desired,  the  measurement  is  very  simple. 
The  resistance  of  the  wires  taken  in  pairs  (Fig.  142),  is  measured 
exactly  as  in  the  case  of  a  metallic  circuit.  From  these  measure- 
ments the  resistance  of  each  wire  may  be  calculated.  For  instance, 
if  wires  i  and  2  taken  together,  measure  4, 500  ohms,  i  and  3  taken 
together  measure  3,750  ohms,  and  2  and  3  taken  together 
measure  4,700  ohms,  then  the  resistance  of  all  three  wires  is 
— 12 — ~— 6>475-  Wire  number  i  then  measures  the  difference 
between  the  resistance  of  all,  and  that  of  numbers  2  and  3  together, 


141 


FIG.  143. 

or  the  differences  between  6,475  an^  4>7oa  Number  i  therefore 
measures  1,775  ohms.  In  the  same  way  wires  numbers  2  and  3  are 
each  found  to  measure  2,725  and  1,975  ohms. 

When  resistance  measurements  are  made  with  the  earth  as  part 
of  the  circuit,  currents  flowing  in  the  earth  may  interfere  with  the 
results  by  entering  the  wire  and  flowing  along  it.  Such  currents  are 
called  earth  currents.  At  exceptional  times,  as,  for  instance,  during 
the  continuance  of  the  so-called  magnetic  storms,  earth  currents  flow- 
ing on  the  wires  may  be  so  strong  that  telegraphing  may  be  carried 
on  without  any  battery  attached  to  the  wires.  When  earth  currents 
interfere  with  the  measurements  made  on  a  grounded  circuit,  the 
tests  must  be  postponed  until  a  more  favorable  opportunity,  if  ad- 
ditional wires  cannot  be  used  in  making  the  measurements  by  the 
last  method  given  above. 

Insulation  measurements  are  made  with  the  line  disconnected 
from  its  ground  plates  (the  line  open,  Fig.  143).  As  a  general  rule, 
the  insulation  resistance  is  higher  than  an  ordinary  Wheatstone 
bridge  will  measure,  and  the  method  explained  in  Lesson  X,  page 
72  is  used.  The  condition  of  the  insulation  of  a  line  from  day  to 
day  may  also  be  roughly  determined  by  means  of  a  milliamperemeter 
(Lesson  XI,  page  75),  which  is  placed  in  the  circuit  at  one  end  of 
the  line,  and  then  a  battery  of  a  fixed  number  of  cells  is  connected 
in  the  circuit  at  the  other  end  of  the  line.  If  the  resistance  of  the 
circuit  and  the  pressure  of  the  battery  be  known,  a  certain  standard  cur- 
rent which  may  be  calculated  according  to  Ohm's  Law  (Lesson  VIII, 
page  5 2),  should  flow  through  the  line  when  the  insulation  is  perfect. 
The  difference  between  the  standard  current  and  that  indicated  by 
the  amperemeter  is  a  measure  of  the  leakage  from  the  line. 

A  comparison  of  the  periodical  measurements  of  conductivity 
and  insulation  shows  whether  or  not  the  line  is  in  good  order,  or 
whether  or  not  any  poor  connections  are  developing  or  its  insulation 
is  deteriorating. 

The  location  of  the  position  of  a  ground  or  a  cross  on  a  line  may 
be  determined  in  various  ways.  If  the  fault  is  a  dead  ground,  a 
measurement  of  the  resistance  of  th?  line  is  made  by  bridge  from  one 
end  of  the  line,  the  other  end  of  the  line  being  open,  (Fig.  144),  and 
the  distance  to  the  ground  is  calculated  at  once  from  the  resistance 
of  the  line  per  mile.  Thus,  suppose  a  line  500  miles  long  ordinarily 
measures  4,500  ohms  or  9  ohms  per  mile,  and  the  resistance  meas- 
ured through  a  dead  ground  is  1,800  ohms,  then  the  ground  is  200 
miles  from  the  station  where  the  measurement  is  made,  since  9  times 


142 


FIG.  144. 

200  is  equal  to  1,800.  When  the  ground  is  only  partial  its  location 
is  not  so  simple,  since  the  resistance  of  the  leakage  path  comes  into 
the  measurements.  Several  methods  may  be  used  in  making  the 
measurements,  but  the  two  following  are  the  simplest.  The  resis- 
tance of  the  line  through  the  fault  may  be  measured  from  each  end, 
the  other  end  being  open  at  the  time.  To  find  the  resistance  of  the 
line  between  one  end,  A,  and  the  fault,  the  resistance  of  the  line  in 
good  order  is  added  to  that  measured  through  the  fault  from  A. 
From  this  is  subtracted  the  resistance  measured  through  the  fault 
from  B  and  the  result  is  divided  by  two.  For  instance,  suppose  the 
resistance  measured  through  the  fault  from  A,  as  shown  in  Fig.  144, 
is  3,500  ohms,  and  a  similar  measurement  made  from  B  shows  5,000 
ohms,  the  line  itself  from  A  to  B  measuring  4,500  ohms, — then,  the 
resistance  of  the  line  from  A  to  the  fault  is  3>50Q+4'f)~5'000^  1,500. 
If  the  line  measures  9  ohms  to  the  mile,  the  distance  from  A  to 
the  fault  is  167  miles.  The  reason  for  this  is  readily  seen,  since 
the  total  resistance  of  the  line  is  equal  to  that  from  A  to  the  fault, 
added  to  that  from  B  to  the  fault,  F.  The  measurement  from  A 
through  the  fault  gives  the  resistance  from  A  to  F  added  to  the 
resistance  of  the  leak.  The  measurement  from  B  through  the  leak 
gives  the  resistance  from  B  to  F  added  to  the  resistance  of  the  leak. 
Adding  together  the  resistance  of  the  line  in  good  order  and  the 
resistance  from  A  through  the  fault,  gives  a  sum  which  is  equal  to 
the  resistance  of  the  leak  plus  the  resistance  of  the  line  from  F  to  B, 
plus  twice  the  resistance  of  the  line  from  A  to  F.  Subtracting  the 
resistance  from  B  through  the  leak  leaves  a  remainder  equal  to  twice 
the  resistance  of  the  line  from  A  to  F. 

The  second  method  of  locating  a  fault  is  by  what  is  called  the 
loop  method.  This  can  be  used  only  when  the  leaky  wire  can  be 
looped  with  a  good  wire  so  that  both  ends  may  be  connected  to  a 
"bridge  for  testing.  In  this  case  the  connection  is  made  up  as  shown 
in  diagram  in  Fig.  145,  where  EP  is  the  leaky  wire  and  CP  is  the  good 
one.  Af  makes  one  bridge  arm  and  Cf  another,  while  A  B  and  B  C 
are  the  other  two  arms.  When  AE  or  A  B  and  A  C  are  adjusted  until 
the  bridge  is  balanced,  the  resistance  from  C  to  f  and  from  A  to  f 
are  to  each  other  as  B  C  is  to  A  B,  while  the  total  resistance  of  Cf 


143 


FIG.  145. 


FIG.  146. 


"YVo* 


FIG.  147- 

plus  Ef  plus  A  E  are  known  from  the  records  of  the  wire  conductiv- 
ities and  the  reading  of  the  rheostat,  A  E.  The  way  in  which  the 
connections  are  made  to  a  postoffice  pattern  bridge  is  shown  in 
Fig.  146. 

When  two  wires  are  crossed,  the  location  of  the  point  where 
they  make  contact  with  each  other  is  carried  out  in  very  much  the 
same  manner  as  the  location  of  grounds,  except  that  the  measure- 
ments are  made  over  a  circuit  made  up  of  the  two  crossed  wires 
(Fig.  147)  instead  of  over  a  circuit  made  up  of  the  ground  and  the 
grounded  wire.  The  distance  from  the  measuring  station  to  the 
cross  is  calculated  from  the  measured  resistance  and  the  resistance 
per  mile  of  the  two  wires  together.  Thus,  suppose  the  resistance 
measured  through  the  cross  at  X,  as  shown  in  Fig.  147,  is  4,400 
ohms,  and  the  resistances  of  the  two  wires  are  9  and  13  ohms  per 
mile.  Then  the  cross  is  200  miles  from  the  measuring  station,  since 
the  resistance  per  mile  of  the  two  wires  together  is  9  plus  13,  or  22 
ohms.  In  this  measurement  it  is  assumed  that  the  resistance  at  the 
cross  itself  is  too  small  to  be  taken  into  account.  When  this  is  not 
the  case,  special  measurements  have  to  be  made  as  in  the  case  of  a 
partial  ground. 

In  making  test  measurements  it  is  usual  to  disconnect  all  telegraph 
or  telephone  instruments  from  the  circuit,  though  they  may  be  per- 
mitted to  remain  in  circuit  and  a  correction  made  on  account  of  theil 
resistance  or  insulation. 


FIG.  148. 

In  testing  underground  wires  and  submarine  cables,  practically 
the  same  methods  are  used  as  in  the  testing  of  overhead  wires. 
Systematic,  periodical  tests  are  quite  essential  for  the  preservation  of 
the  life  of  cables,  since  their  usefulness  may  be  quickly  destroyed  after 
a  leak  starts.  Fig.  148  shows  the  permanent  testing  arrangements  as 
they  are  set  up  in  the  testing-room  at  the  end  of  an  ocean  cable. 

Copyrighted,  1894, 


145 


The  National  School  of  Electricity. 

REVIEW  OF  LESSON  XVIII. 

Points  for  Review.     1.     What  are  the  classes  of  trouble  which  occur  on  telegraph 
and  telephone  lines? 

2.  What  is  a  ground?     What  is  a  cross? 

3.  How  can  the  position  of  trouble  be  located  on  a  local  telegraph  line? 

4.  What  are  earth  currents? 

5.  How  is  the  conductivity  of  a  line  measured? 

6.  How  is  the  insulation  of  a  line  measured? 

7.  How  can  the  position  of  a  ground  be  located  by  electrical  measurements? 

8.  How  can  the  position  of  a  cross  be  located  by  electrical  measurements? 

I^KSSON  XIX. 

PRINCIPLES  OF  CONTINUOUS  CURRENT  DYNAMOS 

AND  MOTORS. 


The  experiments  of  Oersted  (Lesson  VI,  page  35),  Sturgeon 
(Lesson  VI,  page  40),  and  others,  showed  the  intimate  relation 
existing  between  electricity  and  magnetism,  and  also  showed 
that  the  flow  of  an  electric  current  always  produces  magnetism 
(Lessons  VI.  and  VII).  It  remained  for  the  brilliant  experimental 
studies  of  Prof.  Joseph  Henry,  of  Princeton  College,  New  Jersey,  and 
Michael  Faraday,  of  the  Royal  Institution,  London,  to  make  the 
most  important  additions  to  our  knowledge  of  the  mutual  action 
between  electric  currents  and  magnetism.  Within  two  years  after 
the  publication  by  Oersted  that  a  magnetic  needle  may  be  deflected 
by  bringing  near  it  a  wire  carrying  an  electric  current,  Faraday  had 
succeeded  in  producing  a  continuous  motion  by  means  of  the  effect 
of  an  electric  current  upon  a  permanent  magnet,  and  it  was  soon 
after  learned  that  a  wire  hung  over  the  pole  of  a  magnet  and 
with  its  ends  in  mercury  troughs,  as  shown  in  Fig.  149,  would 
continuously  revolve  around  the  pole  on  account  of  the  mutual 
attraction  of  the  lines  of  force  belonging  to  the  magnet  and  to  a 
current  in  the  wire  (Lesson  VI,  page  38).^  In  the  motion  thus  pro- 
duced, by  means  similar  to  those  utilized  in  many  of  the  electrical 
instruments  which  have  already  been  described,  lies  the  principle  of 
the  operation  of  the  electric  motors  which  prove  so  useful  at  the 
present  day.  At  the  time  of  Faraday,  the  best  method  of  generating 
n\an  electric  current  was  by  means  of  an  electric  battery,  but  the  use- 
res'fllness  of  the  electric  motor  could  be  but  small  as  long  as  it  depended 
its  power  upon  the  consumption  of  zinc  in  a  battery  (Lesson  IV, 

24). 

146 


FIG.  149. 


FIG.  150. 


To  the  vigorous  minds  of  Faraday  and  Henry,  the  production  of 
motion  when  an  electric  current  was  brought  into  the  influence  of  a 
magnet,  seemed  to  suggest  a  reverse  action  through  which  an  electric 
current  might  be  produced  by  the  motion  of  a  wire  in  a  magnetic 
field.  This  thought  led,  shortly  after  1830,  to  the  magnificent  dis- 
covery by  Faraday  that  a  tendency  for  electric  currents  to  flow  is  pro- 
duced in  a  conductor  which  is  moved  in  a  magnetic  field  so  as  to  cut 
through  the  lines  of  force  of  the  field.  That  is,  an  electric  pressure  is 
set  up  in  the  conductor  when  it  cuts  the  lines  of  force.  The  two 
great  experimenters  also  independently  discovered  the  fact  that  any 
change  in  the  magnetic  field  around  a  wire  tends  to  set  up  an  electric 
current  in  the  wire,  exactly  as  any  change  in  an  electric  current 
which  flows  in  a  wire  causes  a  corresponding  change  in  the  magnetic 
field  about  it.  In  this  great  discovery  lies  the  principle  of  the  oper- 
ation of  dynamo  electric  generators  or  dynamos,  as  they  are  usually 
called.  Faraday  himself  made  in  1831  what  may  be  called  the  first 
model  of  a  dynamo.  This  consisted  of  a  disc  of  copper  rotated  between 
the  poles  of  a  strong  magnet  (Fig.  150).  From  this  disc  a  current 
was  collected  by  copper  brushes  which  rubbed  on  the  edge  of  the 
disc  and  on  its  shaft. 

Faraday's  discovery  was  quickly  turned  into  commercial  service 
and  many  small  machines  were  made  for  generating  electric  currents 
by  rotating  coils  of  wire  between  the  poles  of  permanent  magnets. 
These  machines  with  permanent  magnets  are  ordinarily  spoken  of  as 
magneto  electric  generators  or  magnetos,  to  distinguish  them  from  the 
ordinary  dynamo  electric  generators  or  dynamos  which  have  electro- 
magnets. The  magnetos  which  are  used  for  ringing  telephone  call  bells 
(Lesson  XVI,  page  122),  belong  to  the  same  class  as  the  early  machines. 

When  a  wire  is  moved  in  a  magnetic  field  so  that  it  cuts 
lines  of  force,  the  action  which  occurs  causes  a  difference  of  electric 
pressure  between  the  two  ends  of  the  wires.  The  magnitude  and 
direction  of  the  pressure  which  is  thus  induced  depends  upon  certain 
fixed  relations. 

The  magnitude  of  the  pressure  depends  upon  the  rate  at  which  the 

147 


FIG.  151.  FIG.  152. 

wire  cuts  lines  of  force,  that  is,  upon  the  total  number  of  lines  of  force 
cut  by  the  wire  in  a  second  of  time.  When  the  wire  cuts  one  hundred 
million  (100,000,000)  lines  of  force  in  every  second  during  its  motion, 
an  electric  pressure  of  one  volt  is  set  up,  and  if  the  wire  (like  C  in  Fig. 
151)  be  laid  across  conducting  rails  which  are  electrically  connected 
through  a  galvanometer  (shown  at  G  in  the  figure)  the  galvanometer 
will  indicate,  while  the  wire  moves,  the  flow  of  a  current,  having  a 
strength  which  is  equal  to  the  induced  pressure  divided  by  the  resist- 
ance of  the  electric  circuit  made  up  of  the  galvanometer,  rails,  and 
moving  wire.  If  the  wire  cuts  through  the  lines  of  force  at  the  rate 
of  two  hundred  millions  (200,000,000)  to  the  second,  the  induced 
pressure  is  equal  to  two  volts,  and  if  the  wire  cuts  only  75  million 
lines  each  second,  a  pressure  of  only  ^  volts  is  set  up,  which  is  ac- 
cording to  the  rule  given  above. 

The  number  of  lines  of  force  which  are  cut  in  a  second  by  a 
wire  moving  in  a  magnetic  field  depends  upon  four  items:  i,  upon 
the  strength  of  the  field,  or  the  number  of  lines  of  force  which  it  con- 
tains in  each  square  centimeter;  2,  upon  the  length  of  the  wire  which 
is  in  the  field;  3,  upon  the  speed  with  which  the  wire  moves;  4,  upon 
the  angle  with  which  the  wire  moves  across  the  lines  of  force.  If 
the  wire  moves  diagonally  across  the  lines  of  force  it  does  not  cut 
through  as  many  lines  in  a  given  time  as  when  it  moves  equally 
fast  at  right  angles  to  the  lines. 

The  direction  of  the  induced  electric  pressure  depends  upon  the 
direction  of  the  lines  of  force  in  the  magnetic  field  and  the  direction  in 
which  the  wire  cuts  through  them.  In  Fig.  152,  if  the  vertical 
arrows  show  the  direction  of  the  lines  of  force  and  the  horizontal 
arrow  between  the  rails  shows  the  direction  in  which  the  wire  A  B 
moves,  then  the  end  B  of  the  moving  wire  is  positive  and  the 
other  end  negative  in  pressure.  That  is,  a  current  will  flow  around 
the  circuit,  composed  of  the  wire  and  the  rails,  from  B  through  C 
and  D  to  A  and  from  A  through  the  wire  to  B.  The  current  flows 
in  the  external  circuit,  B  C  D  A,  from  the  positive  or  high  pressure 
end  to  the  negative  or  low  pressure  end  of  the  wire,  and  within  the 
moving  wire  the  current  flows  from  the  low  pressure  end  to  the  high 
pressure  end.  The  motion  of  the  wire  across  the  lines  of  force 
causes  it  to  act  like  a  pump,  which  lifts  the  electric  current  from  its 
low  pressure  or  suction  end  to  its  high  pressure  or  discharge  end.  In 
this  respect  the  moving  wire  acts  exactly  like  a  friction  machine  or  a 

148 


FIG.  154. 


FIG.  153. 


primary  battery  (Lesson  II,  page  10,  and  Lesson  III,  page  16). 

If  the  direction  of  the  wire's  motion  be  reversed,  the  direction 
of  the  current  will  also  be  reversed.  Reversing  the  direction  of  the 
lines  of  force  also  reverses  the  current. 

There  are  various  ways  of  remembering  the  relation  between 
the  direction  of  the  electric  current,  the  direction  of  the  wire's 
motion,  and  the  direction  of  the  lines  of  force.  One  of  them  is  to 
hold  up  the  right  hand,  with  the  thumb  sticking  straight  up,  the 
first  finger  sticking  straight  out,  and  the  middle  finger  turned  off  to 
the  left  (Fig.  153).  Now,  if  the  hand  be  turned  in  such  a  direction 
that  the  thumb  points  in  the  direction  of  motion  of  the  wire  and  the 
first  finger  points  in  the  direction  of  the  lines  of  force,  then  the  mid- 
dle or  central  finger  will  point  in  the  direction  of  the  current  which 
is  set  up  in  the  wire  by  the  induced  pressure. 

Another  way  of  remembering  this  relation  is  by  a  modification 
of  Ampere's  rule  (Lesson  VI,  page  37).  If  a  man  lies  in  the  mov- 
ing conductor  so  that  he  looks  down  along  the  lines  of  force  (his  face 
is  towards  the  south  pole),  and  the  motion  is  towards  his  right  hand, 
he  will  be  floating  head  first  down  the  current  which  is  set  up  in 
the  wire. 

It  has  already  been  explained  (Lesson  V,  page  31)  that  the  earth 
is  a  great  magnet,  and  that  its  lines  of  force,  therefore,  reach  out 
through  all  the  space  within  which  we  live.  The  induction  of  elec- 
tric pressure  by  a  wire  cutting  lines  of  force  may,  therefore,  be  illus- 
trated by  swinging  a  long  wire  in  the  earth's  magnetic  field.  If  a 
wire  be  suspended  across  a  room  and  its  ends  be  attached  to  a  sensi* 

149 


tive  galvanometer,  the  needle  01  the  galvanometer  will  be  deflected 
from  side  to  side  when  the  wire  is  set  to  swinging.  When  the^wire 
moves  in  one  direction,  the  needle  will  move  to  one  side  of  its  zero 
point;  and  when  the  wire  moves  in  the  other  direction,  the  needle 
will  move  to  the  other  side  of  the  zero.  This  shows  that  the  direc- 
tion of  the  pressure  induced  by  the  cutting  of  the  earth's  lines  offeree 
depends  upon  the  direction  in  which  the  wire  moves  across  the  lines. 

If  the  wire  be  caused  by  some  means  to  swing  more  slowly,  the 
deflections  of  the  galvanometer  needle  will  be  smaller,  showing  that 
the  magnitude  of  the  induced  pressure  depends  upon  the  velocity  of 
motion  of  the  wire. 

If  half  the  wire  be  now  replaced  by  a  piece  of  string,  and  the 
ends  of  the  remaining  half  be  connected  to  the  galvanometer  with- 
out practically  altering  the  resistance  of  the  circuit,  and  the  wire  be 
set  swinging  at  about  the  same  speed  as  before,  the  galvanometer 
deflections  are  reduced  to  about  one  half  their  former  value,  showing 
that  the  induced  pressure  depends  upon  the  length  of  the  wire. 

These  experiments  can  only  be  successfully  carried  out  in  some 
such  favorably  equipped  place  as  a  college  laboratory,  but  their 
description  serves  to  illustrate  the  effect  of  moving  a  conductor 
across  magnetic  lines  of  force.  An  experiment  illustrating  the  same 
thing  may  be  made  by  a  permanent  magnet,  a  coil  of  wire,  and  any 
galvanometer  with  a  light  needle  which  is  obtainable.  If  the  coil 
made  up  of  a  few  turns  of  wire  be  slipped  along  one  end  of  the  magnet 
at  a  fixed  speed,  the  galvanometer  needle  will  show  a  certain  deflec- 
tion. Now  if  more  turns  be  added  to  the  coil,  which  is  then  moved 
exactly  as  before,  the  galvanometer  deflection  will  be  proportionally 
greater,  showing  that  a  greater  electric  pressure  has  been  induced. 
In  the  case  of  the  coil  we  have  the  following  condition;  each  turn 
cuts  the  lines  of  force  at  a  certain  rate  as  the  coil  is  slipped  along 
the  magnet,  and  a  corresponding  electric  pressure  is  set  up  in  it. 
Since  the  turns  of  the  coil  are  all  connected  in  series  and  the  elec- 
tric pressures  set  up  in  them  are  all  in  the  same  direction,  the  elec- 
tric pressure  induced  in  the  whole  coil  is  equal  to  the  sum  of  the 
pressures  developed  in  all  of  its  turns.  This  is  exactly  similar  to  the 
case  of  an  electric  battery  with  its  cells  connected  in  series,  where 
the  battery  pressure  is  equal  to  the  sum  of  the  pressures  of  all  the 
cells.  Adding  additional  cells  to  the  battery  increases  the  battery 
pressure,  and  adding  additional  turns  to  the  moving  coil  increases  the 
total  pressure  induced  in  it. 

If  the  connections  of  some  of  the  cells  in  the  battery  are  reversed, 
the  pressure  at  the  battery  terminals  is  reduced  and  becomes  equal 
to  the  difference  of  the  pressures  which  are  developed  by  the  cells 
connected  in  one  way  and  those  which  are  connected  in  the  reverse 
way.  In  the  same  way,*  if  part  of  the  turns  of  the  moving  coil  be 
wound  in  one  direction  and  part  in  the  other  direction,  the  pressures 


150 


FIG.  155. 


FIG.  156. 

developed  in  the  two  parts  are  opposite,  and  the  effective  pressure 
developed  by  the  coil  is  equal  to  the  difference  of  the  pressures  which 
are  developed  in  the  parts.  If  half  the  turns  are  right  handed  and 
half  left  handed,  no  current  will  flow  in  the  coil  when  it  moves  in 
the  magnetic  field,  because  the  pressure  developed  in  one  half  of  the 
turns  tends  to  cause  the  current  to  flow  one  way,  and  the  equal  pres- 
sure developed  in  the  other  half  of  the  turns  tends  to  cause  the  cur- 
rent to  flow  in  the  opposite  direction.  These  two  tendencies  neu- 
tralize each  other,  and  no  current  flows. 

For  the  same  reason,  if  a  coil  of  wire  be  moved  straight  across  the 
lines  of  force  of  a  uniform  field  (Fig.  154)  no  current  will  flow  in  the 
coil,  since  the  pressures  developed  in  the  two  halves  of  each  turn  are 
in  opposition,  as  shown  by  the  arrows,  and  are  of  equal  value.  The 
truth  of  this  may  be  easily  proved  by  applying  one  of  the  rules  given 
earlier  (page  150).  If  the  coil  be  mounted  on  an  axis  or  shaft,  so  that 
it  may  be  revolved  in  the  field  (Fig.  155),  a  different  condition  exists. 
Now,  the  two  halves  of  the  coil  cut  the  lines  of  force  in  such  a  way 
that  the  pressures  are  in  the  same  direction  as  shown  by  the  arrows, 
and  a  current  therefore  flows  in  the  coil.  Fig.  156  shows  the  coil  after 
it  has  turned  through  a  half  revolution  from  its  first  position.  From 
this  figure  it  is  seen  that  the  two  sides  of  the  coil  are  now  both  cutting 
the  lines  of  foioe  in  a  direction  which  is  opposite  to  that  in  which 
they  cut  the  lines  before.  The  direction  of  the  current  in  the  coil  is 
therefore  reversed.  As  the  coil  continues  revolving  the  current  in 
it  is  reversed  in  every  half  revolution.  Such  a  current,  which  flows 
first  in  one  direction  and  then  in  another,  is  called  an  alternating 
current. 


151 


FIG.  157. 


FIG.  158. 

If,  instead  of  being  short  circuited  on  itself,  the  coil  be  connected 
to  an  external  circuit  by  means  of  such  sliding  contacts  as  are  shown 
in  Fig.  157,  the  alternating  current  may  be  led  off  to  be  used  for  any 
desired  purpose.  The  rings  A  A,  to  which  the  ends  of  the  coil  are 
attached,  in  this  case  are  called  collecting  rings  or  collectors,  and  the 
parts  B  B,  which  bear  on  the  collectors,  are  cabled  brushes.  In  an 
actual  machine  made  up  for  the  purpose  of  generating  electricity  by 
a  coil  revolving  in  a  magnetic  field,  the  revolving  part  is  called  an 
armature.  Telephone  magnetos,  which  have  already  been  referred  to, 
consist  of  a  coil  of  wire  wound  on  an  iron  core,  which  is  revolved  in 
the  magnetic  field  between  the  poles  of  a  horse  shoe  magnet  (Fig. 
158).  Such  machines  produce  an  alternating  current. 


FIG.  159.  FIG.  160. 

It  is  possible  to  arrange  the  collector  which  is  attached  to  a  coil 
that  is  revolved  in  a  magnetic  field  in  the  manner  shown  in  Fig.  159. 
With  this  arrangement,  the  collector  segments  connect  each  brush 
first  with  one  end  of  the  coil  and  then  with  the  other  end  as  the  coil 
revolves.  If  the  brushes  are  properly  set,  that  is,  if  they  bear  on  the 
collector  at  proper  points,  this  arrangement  causes  the  current  to 
flow  continuously  in  one  direction  in  the  external  circuit,  though 
in  the  coil  itself,  its  direction  of  flow  reverses  with  each  half  revo- 
lution as  before.  Such  an  arrangement  of  the  collector  is  called  a 
commutator,  and  the  current  in  the  outside  circuit  is  said  to  be 
commutated  or  rectified.  Fig.  160  shows  one  of  the  early  dynamos 

152 


FIG.  161. 


FIG.  162. 

with  a  single  coil  armature  and  commutatpr  of  two  segments.  This 
machine  looks  quite  like  the  magneto  shown  in  Fig.  158,  but  the 
collector  is  different  and  the  magnetic  field  is  set  up  by  an  electro- 
magnet instead  of  a  permanent  magnet. 

An  armature  with  one  coil  furnishes  a  current  consisting  of  a 
series  of  waves  or  pulsations,  which  may  be  represented  by  Fig.  161. 
This  is  easily  understood  after  a  little  consideration.  When  the  coil 
stands  up  and  down  between  the  pole  pieces  like  the  full  lines  in 
Figs.  155  and  156,  it  is  in  such  a  position  that  when  it  is  revolved  a 
small  amount,  the  conductors  move  practically  parallel  to  the  lines 
of  force  and  no  lines  are  cut.  When  the  coil  is  in  continuous 
revolution,  no  pressure  is  induced  at  the  instant  that  it  is  in  the 
positions  shown  by  the  full  lines  in  Figs.  155  and  156  (A.,  C.  and  E., 
Fig.  161). 

When  the  coil  stands  as  shown  by  the  dotted  lines  in  Figs.  155 
and  156,  it  is  in  such  a  position,  that  when  it  is  moved  a  little,  the 
conductors  cut  squarely  across  the  lines  of  force  and  the  largest  pos- 
sible number  of  lines  of  force  are  cut  for  a  given  amount  of  motion. 
The  dotted  positions  of  the  coil  correspond  with  the  points  B  and  D 
in  fig.  161. 

Direct  current  dynamos  having  armatures  with  one  coil  are  not 
satisfactory  for  general  use  for  two  reasons:  ist,  the  wavy  character 
of  the  current  is  a  disadvantage  for  some  purposes;  ad,  the  commuta- 
tion of  large  currents  at  the  full  pressure  which  is  required  for  most 
commercial  uses  is  not  practical.  To  overcome  these  difficulties  the 
armature  coils  must  be  uniformly  distributed  over  the  surface  of  the 
armature,  and  the  windings  must  be  connected  at  equal  intervals  to 
commutator  segments.  The  first  armature  of  this  kind  that  was  put 
into  commercial  service  was  invented  by  a  Frenchman  named 
Gramme.  The  core  of  Gramme's  armature  consisted  of  a  ring  made 
of  iron  wire.  This  ring  had  a  winding  of  insulated  copper  wire 
wound  uniformly  over  its  surface  and  at  equal  intervals  the  windings 
were  electrically  connected  to  commutator  segments.  The  arrange- 
ment is  shown  in  Fig.  162.  When  this  armature  is  placed  in  a  mag- 


153 


FIG.  163. 


N 


FIG.  164. 

netic  field  the  lines  of  force  pass  through  the  iron  core  from  one  pole 
to  another  (Fig.  163)  so  that  the  revolution  of  the  ring  causes  the 
outer  conductors  to  cut  lines  of  force  but  the  inner  conductors  are 
entirely  shielded.  When  the  armature  is  revolved  the  wires  of  the 
armature  winding  which  are  under  one  pole  piece  cut  lines  of  force 
in  one  direction,  and  those  under  the  other  pole  piece  cut  lines  in 
the  opposite  direction.  The  effect  of  the  opposing  electric  pressures 
which  are  thus  set  up  in  the  windings  of  the  armatures,  is  to  cause  a 
point  at  one  side  of  the  armature  to  come  to  a  high  electrical  pres- 
sure and  a  point  on  the  opposite  side  to  come  to  a  low  electrical 
pressure.  If  brushes  bear  on  the  commutator  at  these  points  (A  and 
B  in  Fig.  164)  a  current  will  flow  in  the  external  circuit  from  the 
high  to  the  low  pressure  side  of  the  armature,  that  is,  from  A  to  B. 
The  path  of  the  current  through  the  armature  itself  is  from  B  to  A, 
through  the  two  halves  of  the  armature  in  parallel.  This  is  plainly 
shown  by  the  figure.  Since  the  number  of  conductors  under  the  pole 
pieces  is  practically  the  same  for  every  position  of  the  armature 
during  the  revolution,  the  armature  produces  a  practically  continuous 
current  when  it  is  continuously  revolved  at  a  uniform  rate,  as  when 
it  is  driven  by  a  steam  engine. 

Copyrighted,  1894, 


154 


The  National  School  of  Electricity. 

REVIEW  OF   LESSON  XIX. 

Points  for  review.     1.     Who  was  Joseph  Henry?     Who  was  Michael  Faraday? 

2.  What  is  the  result  of  moving  an  electric  conductor  in  a  magnetic  field? 

3.  What  is  the  effect  of  changing  the  strength  of  the  magnetic  field  which  is  around 
a.  conductor? 

4.  Upon  what  depends  the  magnitude  of   the  electric  pressure  which  is  induced 
when  a  conductor  is  moved  in  a  magnetic  field? 

5.  Upon  what  does  the  direction  of  the  induced  pressure  depend? 

6.  How    may  the  direction  of  the  current    set    up    by    the  induced   pressure   be 
remembered? 

7.  How  may  the  effect  of  moving  a  conductor  in  a  magnetie  field  be  illustrated? 

8.  What  is  the  result  of   moving  a  coil  of   wire  straight  across  a  uniform  magnetic 
field? 

9.  What  is  the  result  of  revolving  the  coil  in  the  field? 

10.  What  is  an  alternating  current? 

11.  How  may  the  current  induced  in  a  revolving  coil   be   taken   off  for  use  in  an 
external  circuit?  <• 

12.  What  is  the  difference  between  dynamos  and  magnetos? 

13.  What  is  the  purpose  of  a  commutator? 


LESSON  xx. 

PRINCIPLES  OF  CONTINUOUS  CURRENT  DYNAMOS 

AND  MOTORS:  THEIR  CONSTRUCTION,  CARE 

AND  ATTENDANCE. 

As  a  rule,  commercial  Gramme  or  ring  armatures  are  not  wound 
with  a  continuous  wire  but  the  divisions  of  the  armature  windings, 
the  ends  of  which  are  connected  to  adjacent  commutator  segments  or 
bars,  are  wound  as  separate  coils.  This  makes  it  possible  to  insu- 
late the  different  parts  of  the  winding  more  effectively  from  each 
other,  and  thus  prevent  the  current  from  jumping  by  a  short  path,  or 
short  circuiting,  directly  from  one  coil  to  another  instead  of  follow- 
ing all  the  way  around  the  coils.  The  separate  coils  are  connected 
to  the  commutator  segments,  and  to  each  other,  so  that  the  wind- 
ing is  in  effect  the  same  as  though  made  with  a  continuous  wire 
connected  at  intervals  to  the  commutator  segments. 

The  armature  core  may  be  an  iron  cylinder  or  drum,  made  out 
of  discs  of  sheet  iron  laid  together  (Fig.  165),  instead  of  an  iron  ring. 
In  this  case  the  winding  seems  more  complicated,  but  its  general 
plan  is  similar  to  that  of  the  ring  armature.  The  winding  consists 
of  a  number  of  coils  wound  uniformly  over  the  surface  of  the  drum, 
which  are  connected  together  in  such  a  way  that  the  zvinding  is  elec- 
trically the  same  as  though  it  had  been  made  zvith  a  single  long  wire. 

155 


FIG.  165 


FIG.  166. 

The  coils  are  connected  to  the  commutator  bars  exactly  as  in  the 
ring  armature,  and  their  effect  in  producing  electrical  pressure  when 
the  armature  is  revolved  is  just  the  same  as  has  already  been  explained 
in  the  case  of  the  ring  armature.  Armatures  with  drum  shaped  cores 
are  called  Siemens  or  drum  armatures.  A  Siemens  armature  with 
four  coils  is  shown  in  Fig.  166,  from  which  may  be  seen  the  way  in 
which  the  wires  are  wound  on  the  core  and  connected  to  the  commu- 
tator. The  same  figure  shows  one  coil  wound  upon  an  armature  core 
which  is  intended  for  sixteen  coils.  Commercial  armatures  usually 
have  from  thirty  to  one  hundred  coils. 

It  has  already  been  said  that  the  early  Gramme  armature  cores 
were  made  out  of  iron  wire  coiled  up  to  form  a  ring.  In  modern 
machines  the  cores  for  both  Gramme  and  Siemens  armatures  are 
built  up  of  discs,  which  are  punched  out  of  sheet  iron  (Fig.  165). 
These  discs  are  usually  insulated  from  each  other  by  thin  tissue 
paper.  The  object  of  dividing  the  cores  into  discs  or  laminating 
them,  and  of  insulating  the  discs  from  each  other,  is  to  prevent  currents 
from  being  set  up  in  the  core  itself  when  it  is  revolved  in  the  .  mag- 
netic field.  The  rule  that  electric  pressures  are  set  up  when  a  con- 
ductor cuts  lines  of  force,  applies  equally  as  much  to  the  core  of  the 
armature  as  to  the  windings.  Currents  tend  to  flow  in  armature  cores 
from  one  end  to  the  other  at  the  surface,  and  back  again  near  the. 
center  of  the  core.  By  properly  laminating  the  cores  these  currents 
are  nearly  all  prevented,  while  the  passage  of  lines  of  force  all  the  way 
through  iron  from  one  side  of  the  core  to  the  other,  is  not  interfered 
with.  The  great  objection  to  permitting  currents  to  circulate  in 
armature  cores  is  the  fact  that  it  takes  power  to  keep  them  circulat- 
ing, and  all  this  power  is  converted  into  heat  in  the  armature  core, 
and  is  wasted.  Compare  Lesson  VIII,  The  heating  of  the  core  has 

156 


FIG.  167. 

another  disadvantage  since  a  high  temperature  is  likely  to  injure  the 
cotton  and  shellac  insulation  which  is  used  between  the  coils  them- 
selves, and  between  the  coils  and  core.  Even  with  the  best  of  lamina- 
tion a  certain  amount  of  power  is  lost,  and  heating  is  caused,  by  cur- 
rents circulating  in  the  core  discs.  These  currents  are  ordinarily  called 
eddy  currents  because  they  eddy  uselessly  through  the  core,  mfoucault 
currents  after  the  name  of  a  scientist  who  made  some  investigations 
many  years  ago  relating  to  the  generation  of  currents  in  masses  of 
metal. 

There  is  an  additional  cause -of  lost  power  and  heating  in  the 
cores  of  armatures  which  cannot  be  reduced  by  lamination.  This 
seems  to  be  due  to  a  sort  of  friction  between  the  molecules  as  they 
are  caused  to  turn  over  by  the  attraction  of  the  magnetic  field  while 
the  armature  revolves.  Every  time  the  molecules  are  caused  to 
turn  around  under  the  influence  of  a  magnetic  field,  a  certain 
amount  of  power  is  used,  which  is  converted  into  heat;  conse- 
quently, for  every  revolution  of  the  armature  a  certain  amount  of 
power  is  used  and  converted  into  heat.  This  effect  is  called  hys- 
teresis. The  amount  of  power  wasted  and  heat  produced  in  a  core 
on  account  of  hysteresis  depends  upon  the  amount  of  iron  in  tne 
core,  the  number  of  revolutions  made  by  it  in  a  minute,  the  density 
of  magnetism  in  the  iron,  and  the  quality  of  the  iron.  It  may  be 
said  that,  in  general,  the  softer  the  iron  the  less  is  the  loss  due  to 
hysteresis;  consequently,  the  iron  used  in  armature  cores  is  very  soft 
wrought-iron  or  steel  which  has  been  carefully  annealed. 

The  magnetic  field  in  which  the  armature  revolves  is  ordinarily 
produced  by  a  great  electromagnet,  as  has  been  said  in  the  preceding 
lesson.  The  frame  of  the  electromagnet  is  so  arranged  that  it  can 
hold  the  windings  required  to  set  up  the  lines  of  force,  and  in  order 
that  the  lines  may  be  caused  to  pass  through  the  armature  the  poles 
are  arranged  to  embrace  the  armature.  These  expanded  poles  are 
called  polepieces  (P  P  in  Fig.  167),  and  the  whole  of  the  magnet 
frame  is  called  the  field  of  the  machine.  The  parts  of  the  field  upon 
which  the  windings  are  placed  are  often  called  the  field-cores  (m.  m. 
in  Fig.  167.) 

It   is   always   necessary   to   allow   a   certain   amount   of  space 


157 


between  the  pole-pieces  and  the  surface  of  the  armature,  and  in 
addition  a  certain  amount  of  space  is  occupied  by  the  armature  wind- 
ings, so  that  a  considerable  depth  of  non-magnetic  material  exists 
between  the  iron  of  the  pole-pieces  and  the  iron  of"  the  armature  core. 
This  space  is  usually  called  the  air  space  or  gap  (G,  Fig.  167). 

The  number  of  ampere  turns  (L,esson  VI,  page  39)  which  are 
required  to  give  the  magneto-motive  force  which  is  needed  to  set  up 
the  lines  of  force  necessary  to  induce  a  given  electrical  pressure  in  the 
armature  windings  depends  upon  the  reluctance  of  the  armature  core, 
of  the  air  gap  and  of  the  magnet  frame.  Since  there  is  no  insulator  ot 
magnetism,  some  of  the  lines  of  force  which  are  set  up  in  the  field  will 
leak  around  the  armature  instead  of  passing  through  it,  and  the 
cross-section  of  iron  in  the  path  of  the  lines  of  force  through  the 
field  must  be  sufficiently  large  to  hold  these  leakage  lines  as  well  as 
the  useful  ones  which  pass  through  the  armature.  It  is  the  leakage 
or  stray  lines  of  force  which  magnetize  watches  when  they  are 
carried  near  a  dynamo.  In  order  that  the  proportion  of  the  total 
number  of  lines  of  force  that  leak  around  the  armature  shall  be 
as  small  as  possible,  the  reluctance  of  the  air  gap,  which  is  always  a 
considerable  part  of  the  total  reluctance  in  the  magnetic  circuit, 
must  be  made  as  small  as  possible. 

It  is  also  of  advantage  to  make  the  air  space  reluctance,  and 
therefore  the  total  reluctance  of  the  magnetic  circuit  as  small  as  pos- 
sible because  the  number  of  ampere  turns  which  are  required  to  set 
up  the  field  magnetism,  are  thereby  reduced,  and  the  expense  of 
building  the  machine  is  consequently  decreased.  For  this  purpose, 
the  armature  core  is  often  made  toothed  and  the  windings  are  placed 
in  the  slots  or  grooves  between  the  teeth.  It  is  sometimes  thought 
that  placing  the  armature  conductors  in  grooves  between  teeth  in  the 
core  permits  some  of  the  lines  of  force  to  pass  through  the  core  In 
such  a  way  that  they  are  are  not  cut  by  the  conductors  as  the  arma- 
ture revolves.  This  is  a  mistake,  however,  and  armatures  with  the 
conductors  wound  in  slots  give  exactly  the  same  electrical  pressure 
when  revolved  in  a  magnetic  field  as  is  given  by  an  armature  with 
the  same  number  of  conductors  wound  on  the  surface  of  its  core  when 
it  is  revolved  at  the  same  speed  in  a  field  of  the  same  strength^ 

We  have  seen  that  the  operation  of  dynamos  is  a  direct  applica- 
tion of  Faraday's  discovery  that  an  electrical  pressure  is  generated  in 
a  conductor  when  it  is  moved  in  a  magnetic  field.  Electric  motors 
work  on  the  principle  that  a  conductor  carrying  a  current  tends  to 
move  when  placed  in  a  magnetic  field,  on  account  of  the  mutual 
action  of  the  lines  of  force  of  the  field  and  of  the  ciirrent.  The  reasons 
for  these  actions  we  do  not  know,  but  we  know  their  existence  as  the 
result  of  experiment  and  are  able  to  apply  their  results  to  practical 
use.  These  two  principles  are  practically  the  reverse  of  each  other, 
and  the  action  of  dynamos  and  motors  is  therefore  a  reversible  one. 


158 


That  is,  a  machine  which  is  designed  to  be  used  as  a  dynamo  to  gen- 
erate electric  currents  when  driven  by  mechanical  power,  may  usually 
be  used  equally  well  to  generate  mechanical  power  when  driven  as  a 
motor  by  an  electrical  current.  It  is  a  fact  that  the  best  dynamos 
usually  make  the  best  motors,  and  manufacturers  sell  their  standard 
machines  to  be  used  either  as  generators  or  motors.  We  shall  there- 
fore treat  them  as  entirely  similar  in  construction.  It  is  only  when 
the  machines  are  built  to  be  used  for  some  special  purpose  that  they 
cannot  be  conveniently  interchanged  in  their  action. 

The  points  required  in  a  good  dynamo  or  motor  for  general  use 
are  a  powerful  magnetic  field,  which  requires  a  small  magnetic  reluc- 
tance in  the  magnetic  circuit;  as  little  waste  as  possible  of  power  by 
heating,  which  requires  that  the  windings  shall  be  well  designed  and 
that  a  good  quality  of  iron  which  is  well  laminated  shall  be  used  in 
the  armature  core;  and  good  insulation  of  the  windings  from  elec- 
trical contact  with  the  iron  cores  and  of  the  various  turns  of  the 
windings  from  each  other.  L"  a  machine  is  striped  with  gold  paint, 
it  does  not  necessarily  follow  that  it  is  a  well  built  machine.  A  good, 
plain  finish  is  of  advantage  in  an  electrical  machine,  because  it  gen- 
erally shows  the  good  quality  of  the' workmanship  which  is  always  nec- 
essary in  a  satisfactory  machine.  A  good  finish  is  also  desirable  because 
it  quickly  shows  dirt  and  bad  treatment  and  thus  makes  evident  any 
neglect  on  the  part  of  the  dynamo  attendant.  Dirt  and  dampness  are 
two  great  enemies  of  the  insulation  of  dynamos  and  motors,  and  the 
machines  must  therefore  be  kept  perfectly  clean  and  dry  in  order  that 
they  may  operate  well  and  last  indefinitely  without  unnecessary 
repairs. 

When  a  dynamo  armature  is  revolved  in  a  magnetic  field  so  as  to 
produce  a  current,  the  lines  of  force  belonging  to  the  current  are  attract- 
ed by  the  lines  of  force  of  the  field.  This  attraction  tends  to  stop  the 
motion  so  that  power  has  to  be  exerted  to  keep  the  armature  moving, 
and  the  total  electrical  power  produced  is  eaus1  to  the  power  exerted 
on  the  armature  less  that  which  is  lost  by  mechanical  and  magnetic 
friction  (Lesson  VIII,  page  52).  The  useful  electrical  power  which  is 
delivered  by  the  dynamo  to  its  external  circuit  is  less  than  the  total 
electrical  power  generated,  by  the  amount  which  is  lost  in  heating 
the  armature  core  by  eddy  currents  and  in  heating  the  armature  and 
magnet  windings  by  the  useful  currents. 

When  a  machine  is  operated  as  a  motor  by  furnishing  current 
to  it  from  an  external  source,  the  same  losses  exist,  so  that  the 
amount  of  electrical  power  which  must  be  furnished  to  it  is  greater 
than  the  mechanical  power  which  is  taken  from  its  pulley.  When 
the  motor  armature  is  caused  to  revolve  by  the  magnetic  attractions, 
its  conductors  cut  the  lines  of  force  of  the  field,  and  an  electric  pressure 
is  therefore  set  up  in  them.  The  direction  of  this  is  opposite  to  that 
of  the  external  source  which  sends  the  current  through  the  armature. 


159 


FIG.  168.  FIG.  169.  FIG.  170. 

The  electric  pressure  which  is  thus  set  up  in  the  armature  conductors 
of  the  motor  is  called  a  counter  electric  pressure  or  counter  electro- 
motive force.  The  wor^k  which  is  done  by  the  motor  is  dependent 
upon  it,  and  a  useful  electric  motor  which  does  not  produce  a  counter 
electric  pressure  is  as  impossible  of  existence  as  is  a  perpetual  motion 
machine.  Seekers  after  either  are  looking  for  the  impossible. 

Dynamos  may  be  divided  into  three  classes  depending  upon  the 
way  in  which  their  field  magnets  are  wound.  These  are:  i.  Series 
wound  (Fig.  168),  in  which  the  field  winding  is  connected  in  series 
with  the  external  circuit,  and  all  the  current  generated  by  the 
dynamo  passes  through  a  thick  wire  which  is  wound  a  comparatively 
few  times  around  the  field  cores;  2.  Shunt  wound  (Fig.  169),  in 
which  a  field  winding  of  high  resistance  is  connected  in  parallel,  or 
as  a  shunt,  to  the  external  circuit,  and  only  a  portion  of  the  current 
generated  by  the  dynamo  passes  around  the  field  cores  through  a 
great  many  turns  of  fine  wire;  3.  Compound  wound  (Fig.  170), 
which  is  a  combination  of  the  first  two,  so  that  the  fields  are  magne- 
tized in  the  same  direction  by  both  a  shunt  and  a  series  winding. 
If  three  dynamos  of  the  same  size  and  shape  have  fields  wound  in 
the  three  different  ways,  the  number  of  ampere  turns  in  the  magnet- 
izing coils  must  be  the  same  in  each.  Since  the  series  winding 
carries  a  large  current,  the  number  of  times  the  current  must  pass 
around  the  magnet  core  to  make  a  given  number  of  ampere  turns  is 
comparatively  small,  and  the  winding  has  comparatively  few  turns. 
The  shunt  winding  carries  a  comparatively  small  current  and  this 
current  must  therefore  pass  many  times  around  the  core  in  order  that 
it  may  have  the  same  magnetizing  effect  as  the  large  current  passing 
a  few  times  around  the  core.  In  the  compound  winding,  the  number 
of  series  turns  and  of  shunt  turns  must  be  so  proportioned  that  the 
number  of  ampere  turns  made  up  by  both  together  shall  be  the  same 
as  in  the  other  cases. 

The  purpose  for  which  a  dynamo  is  to  be  used,  almost  always 
fixes  the  style  of  its  field  windings.  Series  wound  dynamos  are 
ordinarily  used  for  furnishing  a  current  of  constant  strength  to  arc 
lamps  which  are  connected  in  series  (Fig.  179).  Series  windings  are 


FIG.  180.  &  FIG.  179. 

also  used  on  the  fields  of  street  railway  motors.  Shunt  or  compound 
wound  dynamos  are  used  for  furnishing  the  current  to  incandescent 
lamps  or  electric  motors  which  are  all  connected  in  parallel  (Fig. 
1  80)  between  wires  which  are  kept  at  a  constant  difference  of  press- 
ure, and  shunt  wound  motors  are  commonly  used  to  furnish  power 
for  stationary  purposes.  Compound  dynamos  have  quite  an  advan- 
tage for  furnishing  current  to  be  used  by  electric  motors,  that  is,  for 
power  distribution,  because  they  automatically  keep  the  pressure  con- 
stant through  the  combined  action  of  the  shunt  and  series  field  wind- 
ings. The  pressure  supplied  by  shunt  dynamos  decreases  to  a  cer- 
tain degree,  as  the  current  furnished  by  the  armature  increases,  on 
-<.  ccount  of  the  resistance  of  the  armature,  and  because  the  magnetism 
stt  up  by  the  current  in  the  armature  coils  interferes  with  the  field 
magnetism.  The  magnetizing  power  of  a  series  winding,  of  course 
increases  with  the  current  which  is  furnished  by  the  machine,  and 
the  natural  fall  of  pressure  in  a  shunt  dynamo  may  be  entirely  over- 
come, or  even  reversed,  by  the  addition  of  series  turns.  When  shunt 
dynamos  are  used,  it  is  necessary  to  regulate  the  strength  of  the  field 
magnetism  by  means  of  a  variable  resistance  which  is  connected  into 
the  field  circuit  (Fig.  180).  This  resistance  is  often  called  afield 
rheostat  or  hand  regulator. 

In  order  that  the  number  of  ampere  turns  required  to  set  up  the 
magnetism  in  a  dynamo  shall  not  be  excessive,  it  is  important  to 
make  the  magnetic  reluctance  (Lesson  VI,  page  41)  in  the  path  of  the 
lines  of  force  as  small  as  possible.  On  account  of  this,  the  magnet 
frame  composing  the  magnetic  circuit  of  the  field  is  substantially 
made  of  iron.  In  many  machines  good  wrought  iron  is  used  because 
its  permeability  is  greater  than  that  of  cast  iron,  but  cast  iron  costs 
less  per  pound  than  wrought  iron,  so  that  some  manufacturers  use 
cast  iron  in  the  fields  of  their  machines.  In  this  case  a  greater 


161 


MCTNIVF 

^^C 


162 


FIG.  176. 


163 


FIG.  177. 


FIG.  178. 


13-J.. 


weight  of  cast  iron  is  used  to  make  up  for  its  lower  permeability, 
but  on  account  of  the  smaller  cost  of  cast  iron  the  heavier  machine 
may  not  be  any  more  expensive  than  the  lighter  one  in  which 
wrought  iron  is  used.  Fig.  171  shows  a  very  common  form  of 
machine  in  which  the  fields  are  made  of  wrought  iron,  except  the 
pole  pieces  which  are  of  cast  iron.  In  Fig.  172  is  shown  a  machine 
in  which  the  fields  are  made  wholly  of  wrought  iron.  The  form  of 
these  machines  makes  it  necessary  to  support  the  magnet  frame  and 
armature  bearings  by  a  cast  iron  bed  plate.  The  one  horse-power 
"  Letter  Type  n  generator  of  the  Westinghouse  Company  is  a  machine 
in  which  the  fields  are  wholly  of  cast  iron. 

In  some  machines  the  magnet  frames  are  made  of  very  soft  steel 
castings.  This  metal  has  fine  magnetic  qualities  and  therefore  is 
specially  excellent  for  use  where  light  weight  is  important.  The 
field  of  the  great  2,000  horse-power  dynamo  which  was  used  to  fur- 
nish current  to  the  electric  motors  of  the  Intramural  Railway  at  the 
World's  Fair  and  which  is  now  furnishing  current  to  electric  street 
car  motors,  is  made  of  steel.  Fig.  173  shows  a  street  railway  motor 
with  a  steel  magnet  frame. 

Not  only  does  the  material  of  which  the  frame  of  a  dynamo  is 
made  depend  to  some  extent  upon  the  use  for  which  the  machine  is 
intended,  but  the  form  of  the  machine  is  also  a  matter  of  choice 
which  depends  to  a  considerable  extent  upon  the  object  of  the  ma- 
chine. For  instance,  the  motor  shown  in  Fig.  173  is  iron  clad,  that 
is,  the  steel  frame  surrounds  the  field  windings  and  armature.  This 
arrangement  protects  the  windings  from  danger  of  mechanical  injury 
and  from  the  danger  of  being  splashed  by  water  thrown  by  the  car 
wheels  from  puddles  in  the  street.  Water  will  quickly  ruin  the 
insulating  qualities  of  the  cotton  thread  and  canvas  which  are  largely 
used  to  insulate  the  wires  on  dynamos  and  motors. 

The  commonest  forms  of  dynamos  and  motors  which  are  in 
general  use  are  shown  in  Figs.  171,  172,  173,  174,  175,  176,  177  and 
178.  In  Figs.  177  and  178,  the  fields  have  four  and  eight  poles. 
These  are  called  multipolar  to  distinguish  them  from  the  more  com- 
mon two  pole  or  bipolar  machines.  Multipolar  machines  may  have 
any  number  of  pairs  of  poles  which  their  dimensions  will  admit. 
The  armatures  which  are  used  in  multipolar  machines  are  wound 
upon  the  same  principle  as  those  used  in  bipolar  machines,  which 
have  been  explained.  The  number  of  sets  of  brushes  required  to 
take  the  current  from  the  commutator  of  a  multipolar  machine  is 
commonly  equal  to  the  number  of  poles,  but  sometimes  certain 
special  connections  are  made  in  the  armature,  which  make  it  possible 
to  use  only  two  sets  of  brushes  as  shown  in  Fig.  178.  Machines 
having  the  form  shown  in  Figs.  174  and  175,  are  often  spoken  of  as 
consequent  pole  machines  because  the  lines  of  force  appear  to  enter 
the  armature  from  the  center  of  the  frame.  Nearly  all  dynamos  and 


FIG.  181. 


FIG.  182. 


motors  have  forms  which  are  simply  variations  of  those  shown  here. 

When  a  dynamo  is  started  for  the  first  time,  it  is  necessary  to 
magnetize  its  fields  from  some  other  machine.  The  iron  usually  holds 
sufficient  residual  magnetism  to  afterwards  start  the  machine  into 
operation,  Jand  whenever  started  it  will  quickly  build  up  its  magnet- 
ism to  full  strength.  In  order  that  a  dynamo  may  properly  magnet- 
ize itself,  it  is  necessary  that  the  field  windings  be  connected  to  the 
brushes  so  that  the  current  generated  by  the  residual  magnetism  will 
pass  around  the  fields  in  the  proper  direction.  If  the  connections  be 
made  properly,  but  the  direction  of  rotation  of  the  armature  be  then 
reversed,  the  connections  must  also  be  reversed.  This  is  illustrated 
in  Fig.  181,  which  shows  the  difference  in  the  connections  of  a 
shunt  dynamo  when  the  direction  of  the  armature  rotation  is  re- 
versed. 

The  most  important  detail  to  look  after  when  a  dynamo  is  in 
operation  is  the  condition  and  position  of  the  brushes.  Dynamo  and 
motor  brushes  are  sometimes  made  of  copper,  in  which  case  a  bunch 
of  copper  wires  carefully  laid  up  together  and  soldered  at  one  end,  or 
a  number  of  thin  copper  sheets  laid  together  and  soldered  at  one  end, 
is  commonly  used.  Copper  brushes  usually  touch  the  commutator 
on  a  bevel  (Fig.  182).  Sometimes  carbon  brushes  are  used.  These 
are  usually  blocks  of  copper-plated  carbon  which  touch  the  com- 
mutator either  on  a  bevel  or  radially.  The  brushes  are  held  against 
the  commutator  by  means  of  spring  brush  holders.  When  in  proper 
position  they  are  exactly  opposite  each  other  on  a  certain  diameter  of 
the  commutator  of  a  two  pole  machine.  With  the  brushes  in  the 
proper  position,  a  good  machine  will  usually  deliver  its  current  with 
little  or  no  sparking,  while  the  machine  may  spark  badly  if  the  brushes 
are  in  any  other  position.  The  position  of  no  sparking  may  change 
with  the  load  on  the  machine,  in  which  case  the  brushes  on  a  dynamo 
must  be  moved  forward  as  the  load  increases,  and  the  brushes  on  a 
motor  must  be  moved  backward  under  the  same  conditions. 

Further  questions  relating  to  the  handling  of  dynamos  and 
motors  naturally  enter  into  the  following  lessons. 

Copyrighted,  1894, 


166 


The  National  School  of  Electricity. 

REVIEW  OF  LESSON  XX . 

Points  for  Review.     1.     How  is  a  Gramme  armature  wound? 

2.  How  is  a  Siemens  armature  wound? 

3.  In  what  way  does  the  current  which  is  produced  by  the  operation  of  a  Gramme 
or  Siemens  armature  flow  through  the  armature  itself? 

4.  What  is  the  object  of  laminating  armature  cores? 

5.  What  are  foucault  or  eddy  currents? 

6.  What  is  hysteresis? 

7.  Why  is  a  watch  likely  to  become  magnetized  when  brought  near  a  dynamo? 

8.  Why  are  some  dynamos  more  likely  than  others  to  magnetize  watches  which  are 
brought  near  them? 

9.  What  is  the  fundamental  principle  of  the  operation  of  dynamos? 

10.  What  is  the  fundamental  principle  of  the  operation  of  electric  motors' 

11.  What  is  the  relation  between  generators  and  motors? 

12.  What  points  are  necessary  in  a  good  dynamo? 

13.  Why  is  it  necessary  to  keep  dynamos  clean  and  dry?  *" 

14.  What  is  the  relation  between  the  mechanical  work  done  by  an  electric  motor  and 
its  counter  electric  pressure? 

15.  Why  would  an  electric  motor  which  did  not  develop  a  counter  electric  pressure 
be  useless? 

16.  What  are  the  three  types  of  windings  which  are  placed  on  dynamo  field  magnets? 

17.  For  what  purposes  are  series  wound  machines  ordinarily  used? 

18.  For  what  purposes  are  shunt  wound  machines  ordinarily  used? 

19.  For  what  purposes  are  compound  wound  machines  ordinarily  used? 

20.  What  are  multipolar  machines? 

21.  How  are  compound  and  shunt  machines  regulated? 

22.  What  must  be  done  to  the  field  connections  of  a  dynamo  when  the  direction  of 
rotation  of  its  armature  is  reversed? 

23.  Why  must  the  brushes  of  a  dynamo  which  is  in  operation  be  carefully  looked 
after? 


WESSON  XXI. 

ARC  LIGHTING  AND  ARC  LIGHT  MACHINERY. 

The  arc  lights  which  are  so  much  a  necessity  today  for  illum- 
inating the  streets  of  cities  and  all  large  spaces  which  require  a  high 
degree  of  illumination,  whether  in-doors  or  out,  are  the  direct  com- 
mercial outgrowth  of  another  magnificent  discovery  which  was 
announced  shortly  after  1800.  This  discovery  was,  indeed,  nothing 
less  than  the  possibility  of  producing  the  common  electric  arc.  The 
discoverer  of  the  electric  arc,  Sir  Humphrey  Davy,  an  English  scien- 
tist, exhibited  it  on  a  grand  scale  in  1808  in  a  lecture  before  the 
Royal  Institution  in  London,  when  he  connected  the  electric  circuit 
of  two  thousand  or  more  cells  through  two  pieces  of  charcoal  and 
then  gradually  separated  them.  The  result  was  an  arch  or  "  arc"  of 

167 


dazzling  light  between  the  charcoal  tips  such  as  had  never  before 
been  artificially  produced.  Sir  Humphrey  Davy's  experiments  cre- 
ated a  great  deal  of  interest  but  the  real  usefulness  of  the  electric  arc 
was  not  seen  until  Faraday's  later  discoveries  had  laid  the  founda- 
tion for  the  development  of  the  dynamo  and  the  economical  produc- 
tion of  electricity. 

The  means  for  producing  this  arc  of  light  are  comparatively  sim- 
ple. When  two  pointed  pieces  of  carbon  (made  from  charcoal,  coke, 
etc.)  are  joined  to  opposite  poles  of  the  circuit  from  a  powerful  gen- 
erator of  electricity  and  are  touched  together,  a  current  flows  between 
triem.  Where  their  points  are  in  contact  a  considerable  resistance 
exists,  and  the  points  are  heated  by  the  current  unless  they  are 
pressed  very  tightly  together!  If  the  contact  is  quite  loose  the  points 
become  so  hot  as  to  cause  the  carbon  to  pass  off  as  vapor.  Now  if 
the  carbon  points  be  separated  the  current  will  continue  to  flow 
across  the  space  between  the  points  which  is  filled  with  carbon  vapor, 
forming  the  electric  arc.  Carbon  vapor  is  a  much  better  conductor 
of  electricity  than  air,  and  the  current  can  therefore  be  caused  to 
flow  across  a  space  filled  with  it,  though  it  could  not  readily  be 
caused  to  flow  continuously  through  the  same  space  filled  with 
air. 

It  seems  strange  to  speak  of  the  vapor  of  carbon,  but  the 
temperature  of  the  electric  arc  is  so  great  that  it  boils  and  vaporizes 
the  most  refractory  materials.  The  vaporizing  of  any  material  is 
merely  a  question  of  temperature  and  the  vaporization  of  carbon, 
platinum,  gold,  iron,  copper,  etc.,  in  the  electric  arc  is  just  as  simple 
as  the  conversion  of  water  into  steam  (the  vaporization  of  water) 
over  a  common  coal  fire.  The  vaporization  of  "refractory" 
materials  like  carbon,  platinum,  etc.,  simply  requires  a  much  higher 
temperature  than  that  which  is  reached  by  the  coal  fire  which  is 
amply  sufficient  to  boil  water. 

After  the  electric  arc  has  existed  for  a  little  time  between  the 
carbon  points,  the  points  look  very  much  as  shown  in  Fig.  183. 
Both  points  become  quite  hot  and  give  off  light,  but  the  positive 
point  becomes  much  hotter  than  the  negative,  and  from  it  comes 
the  greater  part  of  the  light  of  the  arc.  In  an  arc  which  is  set  up 
with  a  continuous  current,  carbon  is  carried  off  by  the  current  from 
the  positive  point  and  deposited  on  the  negative  point.  The  posi- 
tive point  therefore  becomes  a  little  hollowed  out  on  the  end  as 
shown  in  the  figure.  This  hollow  is  called  the  crater  of  the  arc. 
As  the  greater  part  of  the  light  of  the  arc  comes  from  this  positive 
end  or  crater,  the  positive  carbon  in  arc  lamps  is  almost  always  put 
at  the  top,  in  order  that  the  light  may  be  thrown  downward.  When 
an  arc  is  set  up  with  an  alternating  current,  both  points  become  some- 
what crater-like  and  light  is  given  off  about  equally  from  the  two 
points. 


168 


FIG.  185. 


FIG   186. 


FIG.  194. 

1G9 


FIG.  188. 


FIG.  189. 


FIG.  191. 


170 


Since  the  arc  is  surrounded  by  air  the  carbon  of  which  the  points 
are  composed  is  gradually  burned  up,  and  it  the  carbons  are  fixed 
in  position  the  arc  gra^s  longer  and  longer  until  its  resistance 
becomes  so  great  that  the  Current  can  not  pass  through  it;  the  current 
then  stops  and  the  arc  goes  out.  Since  carbon  is  carried  away  from 
the  positive  point  and  deposited  on  the  negative  point,  the  former 
wastes  away  at  a  rate  which  is  just  about  double  that  of  the  latter. 

In  order  that  the  electric  arc  may  be  used  for  commercial  light- 
ing an  automatic  device  must  be  used  to  keep  the  carbons  fed 
towards  each  other  as  they  waste  away,  so  that  the  arc  shall  always 
have  the  proper  length.  This  is  included  in  the  mechanism  of  what 
are  known  as  arc  lamps.  These  consist  of  a  case  which  contains 
the  feeding  mechanism,  below  which  is  a  frame  to  support  the  lower 
or  negative  carbon  and  a  glass  shade.  The  feeding  mechanism  has 
two  duties  to  perform:  i.  To  separate  the  carbons,  or  strike  the  arc, 
when  the  lamp  is  thrown  into  circuit;  2.  To  regulate  the  movement 
of  the  upper  or  positive  carbon  downwards  towards  the  negative  one 
as  the  carbons  wear  away.  The  lower  carbon  is  usually  clamped 
solidly  at  the  bottom  of  the  lamp  frame,  and  the  upper  one  is 
clamped  at  the  end  of  a  polished  brass  carbon  rod  the  motion  of 
which  is  controlled  by  the  mechanism. 

Fig.  184  shows  the  familiar  form  of  an  arc  lamp.  Figs.  185  to 
1 88  show  the  mechanism  of  different  lamps. 

The  mechanism  of  the  lamp  is.  usually  caused  to  operate  by  the 
opposing  action  of  two  electromagnets.  The  windings  of  one  of 
these  are  composed  of  few  turns  of  comparatively  coarse  wire  which 
are  connected  directly  into  the  circuit  in  series  with  the  arc.  The 
windings  of  the  other  magnet  are  made  of  many  turns  of  compara- 
tively fine  wire  which  are  connected  as  a  shunt  to  the  arc.  The  two 
electromagnets  may  be  plainly  seen  in  Figs.  185  and  186. 

In  some  lamps,  both  windings  are  put  on  the  same  magnet  as  is 
shown  in  Fig.  187.  The  purpose  of  the  windings  is  the  same  in  the 
two  arrangements  and  may  be  explained  by  reference  to  Figs.  185 
and  1 86.  A  brass  lever  which  runs  across  the  lamp,  carries  an  iron 
armature  or  plunger  at  each  end.  The  armatures  are  in  such  posi- 
tions that  they  are  attracted  by  the  two  electro-magnets,  and  the 
lever  is  attached  to  the  mechanism  which  controls  the  carbon  rod. 
The  lamp  is  trimmed  or  carboned  with  the  tips  of  the  two  carbons 
resting  against  each  other.  When  the  lamp  is  thrown  into  circuit 
the  full  current  of  the  circuit  flows  through  the  series  winding  and 
the  lever  is  lifted  by  the  attraction  of  the  series  magnet.  This  causes 
the  mechanism  to  grip  and  raise  the  carbon  rod  sufficiently  to  strike 
the  arc.  As  the  carbons  burn  away  the  electrical  pressure  between 
their  points  becomes  greater  so  that  the  current  in  the  shunt  coil 
increases.  The  armature  of  the  shunt  coil  is  attracted  more  and 
more  strongly  and  the  lever  is  slowly  tipped  until  the  clutch  releases 


171 


the  carbon  rod  sufficiently  for  it  to  slide  slowly  downward  and  thus 
feed  the  positive  carbon  towards  the  negative.  In  order  that  the 
lamp  may  burn  smoothly  and  quietly  v?it  is  necessary  for  the 
feeding  mechanism  to  keep  the  carbons  at  a  uniform  distance 
apart  while  the  lamp  is  burning.  This  can  only  be  accomplished 
when  the  magnetizing  coils  are  properly  balanced  against  each 
other  and  the  strength  of  the  spring,  which  acts  on  the  lever, 
is  properly  adjusted.  Even  when  all  the  adjustments  are  exactly 
right  arc  lamps  will  not  burn  well  unless  the  carbons  are  of  uniform 
quality.  In  some  arc  lamps,  the  carbon  fod  is  controlled  by  a  clock 
work,  which  in  turn  is  controlled  by  the  differential  magnets  (Fig.  185 
and  1 86,)  while  in  others,  a  simple  clutch  is  caused  to  act  on  the  carbon 
rod  by  the  magnets  (Fig.  187). 

In  another  style  of  lamp  the  differential  action  of  the  magnets  is  not 
utilized,  but  the  pull  of  the  shunt  magnet  is  arranged  to  act  against  the 
force  of  a  spring.  The  winding  of  the  magnets  is  quite  similar  to 
those  already  referred  to,  as  shown  in  Figs.  188  and  189,  in  which 
both  the  lamp  as  a  whole  and  a  diagram  of  the  windings  are 
exhibited.  This  style  of  lamp  is  trimmed  so  that  a  little  space 
remains  between  the  carbon  points.  Its  action  is  as  follows: 

When  the  lamp  is  thrown  into  circuit  the  current  flows  through 
the  series  winding  P  and  thence  through  the  contact  at  N  to  the 
other  side  of  the  lamp.  This  causes  the  magnet  to  attract  the  arma- 
ture A  and  the  hold  of  the  clutch  on  the  carbon  rod  is  released.  The 
upper  carbon  at  once  drops  against  the  lower  one,  thus  throwing  the 
starting  coil  J  into  circuit  in  shunt  with  the  coil  P.  The  starting 
coil  attracts  the  armature  just  above  it  and  opens  the  contact  at  N. 
This  throws  the  shunt  coil  K  and  the  series  coil  P  into  series  with 
each  other,  and  they  form  a  shunt  across  the  poles  of  the  lamp.  The 
large  magnet  is  thus  sufficiently  weakened  to  allow  the  spring  S  to 
raise  the  armature  A  which  actuates  the  clutch  mechanism  and 
strikes  the  arc.  Then  as  the  arc*  increases  in  length  the  electrical 
pressure  between  the  carbons  increases,  the  current  flowing  through 
the  combined  coils  K  and  P  increases  and  the  armature  A  is  suffi- 
ciently attracted  to  slightly  release  the  carbon  rod  and  thus  cause  the 
lamp  to  feed. 

As  a  general  rule,  arc  lamps  are  connected  in  series  (Lesson 
XX,  Fig.  179),  so  that  the  same  current  passes  through  all.  This 
current  is  usually  furnished  by  a  series  dynamo  which  automatically 
keeps  the  magnitude  of  the  current  constant.  The  constancy  of  the 
current  is  a  very  important  element  in  the  proper  regulation  of  the 
lamps.  Nearly  all  arc  lamps  are  now  adjusted  so  that  the  pressure 
required  to  pass  the  current  through  the  arc  is  from  45  to  50  volts. 
If  the  pressure  is  made  smaller  the  arc  becomes  shorter  and  gives  less 
light,  and  it  produces  a  continuous  hissing  or  frying  sound.  If  the 
pressure  is  greater,  the  arcjflames  and  flickers,  which  makes  it  unsat- 


FIG.  195. 


FIG.  196. 


FIG,  190. 


FIG.  197. 


UNIVERSITY 


-173 


FIG.  199. 


FIG.  184.  FIG.  193. 


174 


isfactory.  The  current  used  usually  approximates  9.6,  6.5,  or  4 
amperes.  Arc  lamps  which  are  intended  to  be  used  with  9.6  amperes 
are  usually  spoken  of  as  2,000  candle-power  or  450  watt  lamps,  while 
those  intended  to  be  used  with  6. 5  and  4  amperes  are  usually  called 
1, 200  candle-power  and  600  candle-power  lamps.  A  candle  power  is 
equal  to  the  light  given  off  by  a  sperm  candle  of  fixed  size  and  form. 
The  actual  candle  power  given  off  by  the  lamps  is  much  less  than 
these  figures,  and  in  fact  the  light  given  off  in  different  directions 
varies  from  a  hundred  candle-power  or  thereabouts  to  nearly  the  rated 
value  of  the  lamp.  Fig.  190  shows  by  the  curve  the  amount  of  light 
given  off  by  arc  lights  in  different  directions  when  using  various 
currents.  The  greatest  amount  of  light  is  given  off  at  an  angle  of 
about  45°  from  the  direction  of  the  carbons.  For  this  reason  the 
best  effect  may  be  gained  from  arc  lights  used  in  illuminating  streets  by 
hanging  them  from  25  to  35  feet  from  the  ground  over  the  center  of 
the  streets,  or  mounting  them  at  street  corners  on  tall  poles  such  as 
that  shown  in  Fig.  191.  Inside  of  buildings  they  are  usually  hung 
from  small  boards  fastened  to  the  ceiling-.  A  switch  similar  to  that 
shown  in  Fig.  192  is  usually  placed  in  arc  lighting  wires  where  they 
enter  a  building. 

As  the  carbons  which  are  ordinarily  used  in  arc  lamps  are  of 
such  a  length  that  they  will  only  burn  for  seven  or  eight  hours, 
double  lamps  (Fig.  193)  are  used  for  all  night  lighting.  These  con- 
sist of  a  modified  mechanism  which  controls  two  carbon 
rods,  one  of  which  does  not  come  into  service  until  the  car- 
bon held  in  the  first  has  burned  out.  The  carbons  that  are 
ordinarily  used  vary  from  ^  to  ^  inches  in  diameter  and  are 
usually  coated  with  copper  to  reduce  their  resistance.  The  positive 
carbon  is  about  twelve  inches  long  and  the  negative  is  about  six 
inches  long.  The  carbons  are  made  from  finely  ground  coke  or 
lampblack  which  is  mixed  with  syrupy  compounds  and  then  baked 
in  moulds.  The  copper  coat  is  put  on  by  electroplating.  Some- 
times oval  carbons  about  one  inch  broad  and  a  half  inch  thick  are 
used  in  single  lamps  for  all  night  burning. 

The  number  of  successful  manufacturers  of  arc  light  machinery 
is  comparatively  small.  The  earliest  to  enter  the  business  in  this 
country  with  commercial  success  was  the  Brush  Electric  Co.  To 
this  company  is  probably  due  the  introduction  of  lamps  with  differ- 
ential magnets,  which  are  still  so  much  used.  The  Brush  arc 
dynamo  is  shown  in  Fig.  194.  'Figs.  195,  196,  197  and  198  show- 
respectively  the  Thomson-Houston,  Wood,  Standard,  and  Western 
Electric  arc  dynamos.  The  regulation  of  each  of  these  is  performed 
by  moving  the  brushes  around  the  commutator,  so  that  as  lamps  are 
cut  into  and  out  of  circuit  the  pressure  is  varied  so  that  the  current 
is  always  kept  of  constant  value. 

In  order  that  the  dynamos  in  an  arc  light  generating  station 


175 


may  be  properly  managed,  it  is  necessary  to  have  some  arrangement 
by  which  any  dynamo  in  the  station  may  be  connected  to  any  one  of 
the  circuits  which  run  out  to  the  lamps.  The  number  of  dynamos 
and  circuits  may  be  quite  large  in  a  plant  which  is  located  in  a  large 
city.  The  arrangement  that  is  usually  used  for  the  purpose  is  a 
switch-board  (Fig.  199)  fitted  with  a  heavy  spring  jack  for  each  wire 
leading  to  the  dynamos  and  another  similar  spring  jack  for  each  wire 
leading  to  the  lamp  circuits.  The  spring  jacks  may  be  connected  as 
desired  by  plugs  and  cords,  very  much  as  a  telephone  operator  con- 
nects two  subscribers  (Lesson  XVII,  page  135).  The  figure  shows  a 
switchboard  arranged  for  three  lamp  circuits  marked  i,  2,  3,  and  for 
three  dynamo  circuits  marked  A,  B,  C.  Each  dynamo  is  shown  to 
be  connected  to  a  lamp  circuit  by  means  of  plugs  and  cords.  The 
amperemeters  at  the  top  of  the  switchboard  are  connected  in  the 
dynamo  circuits  and  serve  to  show  the  dynamo  attendant  whether 
or  not  the  machines  regulate  properly. 

Copyrighted,  1894, 


176 


The  National  School  of  Electricity. 

REVIEW  OF    LESSON   XXI. 

Points  for  Review:     1.     What  is  the  electric  arc? 

2.  Why  was  the  arc  not  put  into  service  for  commercial  illumination  immediately 
after  its  first  production? 

3.  What  are  arc  lamps? 

4.  What  is  the  principle  of  operation  of  arc  lamps? 

5.  How  are  arc  lamps  usually  connected  in  circuit  and  what  kind  of  a  dynamo  is 
used  to  produce  the  current  supplied  to  them? 

6.  How   much   current  is  used  in  the  ordinary  arc  lamps,  and  what  pressure  as 
required  for  each  arc? 

7.  Why  are  arc  lamps  which  are  used  for  street  illumination  placed  at  a  consider- 
able height  from  the  ground? 

8.  How  are  arc  light  carbons  made? 

9.  What  is  the  object  of  the  regulator  on  arc  light  dynamos? 
10.     How  are  arc  light  switchboards  generally  arranged? 


XXII. 

INCANDESCENT  LIGHTING  AND   POWER  TRANSMIS- 
SION: TWO,  THREE  AND  FIVE  WIRE  SYSTEMS 
OF  DISTRIBUTION  FOR  ELECTRIC 
LIGHTS  AND  MOTORS. 

Illumination  by  arc  lights  is  very  satisfactory  in  streets  or  open 
spaces  out  of  doors  or  in  large  rooms  such  as  shops  or  halls,  but  its 
intense  brilliancy  causes  it  to  cast  dense  shadows  which  totally  unfit 
it  for  satisfactory  use  in  general  indoor  lighting.  Its  unavoidable 
flickering  and  occasional  hissing  also  make  it  unsatisfactory  for 
general  use  in  small  rooms.  If  the  faults  of  the  arc  when  used  for 
general  indoor  lighting  were  not  so  evident,  the  use  of  small  arcs  in 
office  and  house  lighting  might  have  been  attempted  as  early  as  1880, 
by  which  time  the  arc  lamp  had  begun  to  prove  its  value  for  outdoor 
lighting. 

By  1880  the  disadvantages  of  the  arc  for  general  illumination 
had  become  known  and  inventors  were  using  every  effort  to  find 
some  substitute.  Many  years  earlier,  inventors  had  made  electric 
lamps  which  consisted  of  a  loop  of  wire  made  of  platinum  or  iridium, 
two  metals  which  melt  only  at  exceedingly  high  temperatures,  and 
in  which  the  light  was  produced  by  heating  the  wire  white  hot,  or 
to  incandescence,  by  means  of  a  current.  The  light  was  therefore 

177 


produced  by  means  of  the  great  heat  caused  in  the  wire  when  a  cur- 
rent flowed  through  the  high  resistance  of  the  wire  (L,esson  VIII, 
page  53).  This  is  a  case  where  the  C2  R  loss  was  turned  to  a  useful 
account  but  the  lamps  were  not  successful,  though  the  same  principle 
is  used  in  the  incandescent  lamps  of  today.  Just  previous  to  1880 
many  prominent  inventors,  including  Edison,  Maxim,  Farmer,  and 
Sawyer  and  Man  in  this  country,  and  Swan  in  England,  were  mak- 
ing every  effort  to  construct  a  satisfactory  lamp  to  operate  by  the 
incandescence  of  some  material.  It  was  found  that  loops  of  platinum 
and  iridium  were  unsatisfactory  because  they  soon  melted  or  gave 
out  when  continuously  subjected  to  the  high  temperature  which  is 
necessary  to  produce  a  satisfactory  light.  The  only  conducting 
material  which  would  stand  the  high  temperature  of  incandescence 
was  found  to  be  carbon.  Unfortunately  carbon  burns  away  when 
heated  to  a  high  temperature  in  the  air,  and  therefore  could  not  be 
used  in  a  lamp  in  the  same  way  that  the  metallic  wires  had  been. 

As  early  as  1845  a  lamp  had  been  made  in  which  a  thin  stick  of 
carbon  was  enclosed  in  a  glass  globe  from  which  the  air  had  been 
exhausted.  This  lamp  produced  an  excellent  light,  as  the  carbon 
could  not  burn  away  in  a  vacuum,  however  hot  it  became,  but  no 
satisfactory  arrangements  then  existed  for  making  proper  carbon 
sticks  or  for  exhausting  the  air  from  the  glass  globes.  Shortly 
before  1880  the  inventors  turned  from  their  efforts  to  [make  a  satis- 
factory loop  from  a  metal  wire,  to  make  another  attempt  to  use  carbon. 
By  1880  Edison,  Sawyer  and  Man  and  Swan  had  made  lamps  which 
produced  light  through  the  incandescence  of  a  thin  strip  or  filament 
of  carbon.  The  lamp  made  by  Edison  looked  very  much  like  the 
incandescent  electric  lamps  of  the  present  day,  and  it  is  no  doubt  to 
his  industry  and  ingenuity  that  we  owe  the  cheap  and  economical 
form  of  incandescent  lamp  which  we  now  use.  One  of  Edison's 
early  lamps  is  shown  in  Fig.  200.  The  globe  or  bulb  of  the  lamp 
contained  a  filament  of  carbonized  paper  in  an  arched  or  horse-shoe 
form.  The  ends  of  the  carbon  horse-shoe  were  connected  to  short 
pieces  of  platinum  wire  which  passed  through  the  glass  of  the  bulb. 
By  means  of  these  wires  current  could  be  led  to  the  filament.  The 
bulb  was  exhausted  (that  is,  the  air  was  removed)  by  means  of  a 
form  of  mercury  air-pump,  which  is  used  in  a  modified  form  for  the 
same  purpose  at  the  present  day,  and  which  is  capable  of  producing 
a  very  perfect  vacuum. 

Figs.  201  and  202  show  the  two  forms  of  air-pumps  which  have 
been  commonly  used  in  exhausting  lamps.  These  are  often  called 
vacuum  pumps  because  they  are  used  to  produce  a  vacuum.  The 
first  is  called  the  Geissler  pump  after  its  inventor,  who  was  also  the 
maker  of  the  vacuum  tubes  known  as  Geissler  tubes,  which  display 
such  pretty  color  effects  when  an  electric  spark  is  passed  through 
them.  The  pump  shown  in  the  second  figure  is  called  a  Sptengel 


178 


FIG.  200. 


FIG.  201. 

pump,  also  after  the  name  of  its  inventor.  The  Geissler  pump  may 
be  briefly  described  as  an  air  pump  made  of  glass  in  which  mercury  is 
used  as  a  plunger  in  order  that  leakage  of  air  may  be  entirely  avoided. 
In  Fig.  201,  B1  and  B2  are  two  glass  bulbs  which  are  connected  by  a 
long  U  of  glass.  From  the  tube  just  below  B2,  a  tap  leads  off  to  the 
lamps  which  are  to  be  exhausted.  In  this  tap  is  a  valve,  C,  which 
closes  by  being  pushed  upwards,  and  a  bulb,  D,  containing  some 
material  which  absorbs  all  the  moisture  from  the  air  which  passes 
through  it.  This  is  used  because  it  is  necessary  to  keep  the  pump 
perfectly  dry  and  the  air  in  the  lamps  before  they  are  exhausted 
always  contains  some  moisture.  Now,  if  by  means  of  suction  at  P, 
the  mercury  is  caused  to  rise  up  in  B2,  it  pushes  all  the  air  out  of 
B2  through  the  valve  V.  The  mercury  is  prevented  from  reaching 
the  lamps  by  the  valve  C.  When  the  mercury  is  caused  by  suction 
at  I  to  drop  down  again  to  its  old  level,  the  valve  V,  as  the  mercury 
leaves  it  falls  back  into  its  seat,  so  that  no  air  can  get  in  and  the 
bulb  B2  is  left  entirely  free  from  air.  It  is  therefore  ready  to 
receive  a  new  supply  of  air  from  the  lamps  when  the  mercury  level 
falls  below  the  level  of  the  lamp  tap.  The  operation  of  alternately 
exhausting  the  bulb  B2  and  putting  it  into  connection  with  the 
lamps  from  which  it  receives  a  new  portion  of  air,  is  continued  until 
the  lamps  are  properly  exhausted.  The  glass  tube  to  which  the 
lamps  are  connected  is  then  melted  or  sealed  off  at  T,  and  the  lamps 
are  finished.  It  is  this  sealing  off  from  the  pump  that  causes  the 
sharp  tip  at  the  top  of  commercial  incandescent  lamps. 


179 


FIG.  206. 


FIG.  202. 


The  operation  of  the  Sprengel  pump  (Fig.  202),  is  quite  similar 
in  principle  to  the  operation  of  some  injectors.  The  mercury  is 
allowed  to  flow  in  a  jet  through  the  nozzle  J,  and  air  is  drawn  from 
the  lamps  by  the  suction  of  the  drops  of  mercury  rushing  past  the 
end  of  the  lamp  tap. 

The  carbon  filaments  of  incandescent  lamps  are  now  usually 
made  from  bamboo  strips  or  from  silk  or  cotton  threads.  These  are  con- 
verted into  carbon  by  baking,  in  very  much  the  same  way  that  wood 
is  converted  into  charcoal  in  a  kiln.  The  material  is  first  made  into 
exactly  the  proper  size  to  produce  a  filament.  After  proper  treatment 
it  is  then  bent  around  blocks  of  carbon  and  is  packed  in  a  crucible 
filled  with  powdered  carbon.  After  baking  for  many  hours  the 
material  is  converted  into  black  carbon  hairpins,  the  hairpin  form 
coming  from  the  form  of  the  blocks  around  which  the  material  was 
wrapped.  To  bring  the  filaments  to  the  proper  resistance  and  at  the 
sanie  time  put  them  into  condition  to  stand  the  strain  of  the  high 
temperature  of  "burning,"  they  are  commonly  "treated"  by  a  process 
which  deposits  very  hard  grey  carbon  upon  their  surfaces.  The 
filaments  are  then  each  mounted  upon  two  short  pieces  of  platinum 
wire  which  are  sealed  into  a  bit  of  glass.  The  connection  between 
the  carbon  and  the  platinum  is  usually  made  satisfactory  from  an 
electrical  point  of  view  by  means  of  a  cement.  The  filament  thus 
mounted  is  sealed  into  the  bulb  by  a  glass  blower.  The  bulbs  are 
usually  purchased  ready-made  from  a  glass  factory.  One  of  these 


180 


FIG.  208. 


FIG.  209. 


bulbs  is  selected  and  a  piece  of  glass  tube  is  connected  to  the  top  of 
the  bulb.  This  serves  as  a  handle  for  the  workmen  and  also  for  con- 
necting the  lamp  to  the  pump.  The  carbon  is  then  inserted  into  the 
neck  of  the  bulb  and  the  glass  at  the  base  of  the  carbon  is  so  care- 
fully welded  into  the  glass  base  of  the  bulb  that  the  union  becomes 
absolutely  perfect.  After  exhausting,  as  already  explained,  the  lamp 
is  complete. 

For  convenience  in  use,  incandescent  lamps  are  mounted  on 
bases  to  which  they  are  fastened  with  plaster.  These  bases  contain 
two  contacts  which  correspond  to  two  contacts  in  a  socket  which  may 
be  connected  to  an  electric  circuit.  In  Fig.  203,  b  is  the  lamp  bulb, 
c  is  the  carbon  filament,  w  w  are  the  platimum  leading  in  wires,  j  is 
the  cement  connecting  the  carbon  and  platinum,  f  is  the  brass  base 
which  is  attached  to  the  lamp  by  the  plaster  p,  d  and  r  are  the  two 
contacts  by  which  the  carbon  is  brought  into  connection  with  the 
electric  circuit  when  the  lamp  is  inserted  in  a  socket,  and  t  is  the 
point  where  the  lamp  was  *  'sealed  oft' '  the  pump. 

The  bases  used  on  lamps  have  various  external  forms  depending 
upon  the  manufacturer.  Certain  forms,  known  as  the  Edison, 
Thomson-Houston,  and  Sawyer-Man,  which  have  come  into  very 
general  use,  are  shown  in  Figs.  204,  205  and  206.  The  Westing- 
house  lamp,  which  is  shown  in  Fig.  207,  has  a  glass  stopper  in  which 
the  leading-in  wires  are  fixed,  which  is  not  welded  or  sealed  fast  to 
the  bulb,  but  the  long  joint  between  the  stopper  and  the  neck  of  the 
bulb  is  ground  until  it  fits  so  closely  that  it  is  air  tight.  As  an  addi- 
tional precaution  the  outside  of  the  joint  is  covered  with  cement. 

In  order  that  incandescent  lamps  may  be  as  conveniently  turned 
on  and  off  as  gas  lights,  the  sockets  often  contain  switches  as  shown 
in  Fig.  208,  which  represents  a  socket  with  its  brass  shell  removed. 


181 


FIG.  207. 

Where  lamps  are  arranged  to  be  controlled  by  wall  switches,  plain  or 
keyless  sockets  are  generally  used  (Fig.  209). 

Incandescent  electric  lamps  and  electric  motors  are  sometimes 
operated  upon  series  circuits,  but  they  are  much  more  satisfactory  when 
connected  in  parallel  (L,esson  XX,  Fig.  180)  as  is  usually  done. 
The  difference  between  the  connection  of  lamps  in  parallel  and  lamps 
in  series  may  be  illustrated  by  comparing  the  methods  of  utilizing 
water  power.  Suppose  a  series  of  dams  is  placed  in  a  stream  and  a 
mill  is  placed  at  each  dam.  The  water  which  passes  through  the 
waterwheels  of  the  first  mill  flows  down  to  the  second  mill  and  passes 
through  its  wheels  and  thus  continues  to  flow  through  the  wheels  of 
one  mill  after  another.  The  wheels  of  each  mill  are  therefore  turned 
by  the  same  water  that  turns  the  wheels  of  every  other  mill.  In 
order  that  this  may  be  the  condition,  each  mill  must  be  located  on  a 
lower  level  than  the  one  up  stream  from  it.  Then  the  total  fall  of  the 
•stream  is  so  divided  that  each  mill  gets  advantage  of  a  proper  portion. 
In  series  arc  lighting  the  same  current  flows  through  all  the  lamps 
one  after  the  other,  and  the  total  pressure  at  the  dynamo  is  divided 
amongst  the  lamps.  If  an  arc  dynamo  is  capable  of  producing  1,000 
volts  it  will  operate  twenty  lamps  in  series  since  it  takes  about  50 
volts  to  send  the  current  through  each  arc.  If  a  portion  of  the  lamps 
are  cut  out  of  circuit,  the  pressure  at  the  dynamo  must  be  reduced  or 
the  current  will  increase  above  its  proper  value. 

If  a  large  dam  is  built  on  the  stream,  and  the  mills  are  located 
so  that  they  all  take  water  from  the  same  canal  and  discharge  water 


182 


FIG.  210. 

into  the  same  tailrace,  the  water  of  the  stream  is  divided  between 
the  mills  in  proportion  to  their  needs,  and  their  wheels  are  in  par- 
allel. The  amount  of  water  flowing  through  the  wheels  of  each  mill 
in  this  case  is  directly  proportional  to  the  work  being  done  in  the 
mill.  If  one  mill  is  shut  down  the  gate  through  which  water  Is 
admitted  to  the  wheel  is  closed,  and  no  water  flows  through.  The 
water  used  by  each  mill  is  entirely  independent  of  the  amount  used 
by  the  others.  In  the  same  way,  when  electric  lamps  are  connected 
in  parallel  the  current  flowing  through  each  lamp  is  entirely  inde- 
pendent of  that  flowing  through  the  others,  and  simply  depends  upon 
the  resistance  of  the  lamp  and  the  pressure  at  its  terminals.  When 
it  is  desired  to  cut  out  of  circuit  a  lamp  which  is  {connected  in  par- 
allel with  others,  its  connection  with  the  circuit  is  Broken  by  a 
switch  (Fig.  208)  so  that  no  current  can  flow  through  it.  This  is 
equivalent  to  closing  the  gate  through  which  water  enters  a  mill,  as 
already  explained. 

When  it  is  desired  to  shut  down  one  of  a  number  of  mills  in 
series,  it  evidently  will  not  do  to  simply  close  the  gates  which  admit 
water  to  the  wheels,  as  that  would  prevent  the  water  from  flowing  to 
the  other  mills,  but  it  is  necessary  to  arrange  a  short  path  for  the 
water  to  flow  around  the  mill  which  is  shut  down.  In  the  same 
way  when  it  is  desired  to  turn  off  an  electric  lamp  which' is  operated 
in  series  circuit,  the  lamp  must  be  short  circuited  (Fig.  210).  Some 
special  switches  used  on  arc  lighting  circuits  (Lesson  XXI,  Fig. 
192)  short  circuit  the  lamp  which  is  to  be  turned  off,  so  that  the  main 
line  is  properly  completed,  and  then  disconnect  the  lamp  terminals 
from  the  line. 

Since  the  current  which  flows  through  incandescent  lamps  con- 
nected in  parallel  depends  upon  the  pressure  at  the  lamp  terminals, 
and  the  light  given  by  each  filament  depends  upon  the  current  flow- 
ing through  it,  the  pressure  at  the  terminals  of  the  lamps  must  be 
kept  perfectly  constant  or  they  will  not  give  a  steady  light.  If  the 
electrical  pressure  at  the  terminals  of  an  incandescent  lamp  is  changed, 

183 


the  light  given  off  by  the  filament  changes  at  a  much  faster  rate.  If 
a  lamp,  for  instance,  which  is  intended  for  a  pressure  of  no  volts  and 
to  give  1 6  candle-power,  be  connected  to  a  105  volt  circuit,  the  light 
which  it  gives  is  no  more  than  about  12  candle-power  and  is  of  a 
poor  red  color.  If  the  same  lamp  be  connected  to  a  115  volt  circuit, 
the  light  which  it  gives  becomes  about  20  candle-power  and  is  of  a 
brilliant  whitish  color.  The  great  candle-power  and  whiteness  of 
the  light  in  the  latter  case  shows  that  the  filament  is  so  excessively 
hot  that  even  refractory  carbon  cannot  last  long  under  the  strain,  and 
the  filament  will  soon  give  out.  The  length  of  time  during  which 
the  filament  of  an  incandescent  lamp  will  last — that  is,  the  life  of 
the  lamp — decreases  very  rapidly  as  the  temperature  at  which  the 
filament  burns  is  increased  above  its  proper  value.  On  the  other 
hand,  the  power  required  to  produce  light  increases  as  the  working 
temperature  of  the  filament  decreases.  It  should,  therefore,  always 
be  the  aim-  to  work  incandescent  lamps  at  the  exact  pressure  for  which 
they  were  designed. 

We  have  already  seen  that  there  is  always  a  loss  of  pressure  when 
an  electric  current  flows  through  a  wire,  this  loss  being  equal  to  the 
product  of  the  amperes  of  current  and  the  resistance  of  the  wire 
(L<esson  VII,  page  42).  Consequently,  when  a  number  of  incandescent 
lamps  in  parallel  are  connected  to  a  circuit  at  some  distance  from  the 
dynamo  which  supplies  the  current,  the  wires  of  the  circuit  must  be 
quite  heavy  in  order  that  the  loss  in  pressure  shall  not  be  too  great. 

When  incandescent  lamps  and  motors  are  connected  to  wires  which 
lead  from  a  central  generating  station,  it  is  common  to  allow  a  loss 
of  pressure  or  "drop"  amounting  to  as  much  as  ten  percent  to  twenty 
per  cent  of  the  dynamo  pressure  when  all  the  lamps  are  turned  on. 
The  circuits  must  be  svo  arranged  that  all  the  lamps  shall  be  fed  with 
current  at  as  nearly  the  same  pressure  as  possible,  and  if  the  drop 
is  allowed  to  be  greater  than  twenty  per  cent,  this  becomes  a  difficult 
matter.  When  the  drop  in  the  wires  is  too  great,  it  is  also  difficult 
to  regulate  the  dynamo  so  that  the  pressure  at  the  lamps  shall  not 
vary  when  lamps  are  turned  on  or  off.  It  is  easy  to  see  that  every 
lamp  which  is  turned  on  or  off  changes  the  current  flowing  through 
the  wires  of  the  circuit,  and  therefore  changes  the  pressure  lost  in  the 
wires  between  the  dynamos  and  lamps.  In  plants  which  are  confined 
to  a  single  building  the  drop,  when  all  the  load  (lamps  and  motors) 
is  turned  on,  is  usually  made  to  be  from  five  to  ten  per  cent  of  the 
pressure  at  the  dynamo. 

On  page  43  of  Lesson  VII,  it  is  stated  that  the  resistance  of  a  wire 
is  directly  proportional  to  its  length  and  inversely  proportional  to  the 
square  of  its  diameter.  The  diameters  of  wires  are  usually  measured 
in  thousandths  of  an  inch  or  mils,  and  the  square  of  the  diameter  of  a 
wire  when  it  is  given  in  mils  is  called  the  cross  section  of  the  wire 
in  circular  mils.  The  resistance  of  a  commercial  wire  which  is  one 


184 


foot  long  and  has  a  cross  section  of  one  circular  mil  is  about  10.5 
ohms  at  ordinary  temperatures.  The  resistance  of  any  copper  wire  is 
therefore  equal  to  10.5  times  its  length  in  feet  divided  by  its  cross 
section  in  circular  mils.  As  a  formula  which  is  easy  to  remember 
this  may  be  written  ^R=^~^-  (Tables  giving  the  sizes  of  wires  for 
electric  lighting  nearly  always  have  a  column  which  shows  their 
cross-sections  in  circular  mils.)  In  electric  lighting  and  power  dis- 
tribution, the  current  goes  out  from  the  dynamo  to  the  lamps  and 
motors  on  one  wire  and  back  on  another  of  equal  length,  so  that  the 
total  length  of  the  wire  in  the  circuit  is  twice  the  distance  along  the 
line  from  the  dynamo  to  the  lamps.  We  may  put  twice  the 
distance  in  place  of  the  length,  or  2D  in  place  of  L,  and  we  have 
^__ axioox  __2ix_D.  -^ow^  ^e  drop  of  pressure  in  a  line  of  resistance  R 

when  current  C  flows  through  it,  is  v^RxC=81x^xa  This  finally 
shows  that  the  circular  mils  cross  section  required  in  a  wire  which  is 
intended  to  carry  a  current  through  a  circuit  of  a  given  length  with  a 
fixed  number  of  volts  drop  is  M—  21x^xc-  This  may  be  stated  in  the 
following  way:  The  wire  which  conveys  the  current  from  dynamo  to 
lamps  must  have  as  many  circular  mils  in  its  cross  section  as  is  equal 
to  twenty-one  times  the  Distance  from  dynamo  to  lamps  multiplied  by 
the  Current  in  amperes  and  divided  by  the  Drop  in  volts. 

As  an  example,  suppose  that  it  is  desired  to  find  the  size  of  wire 
in  the  Brown  &  Sharpe  (B.  &  S. )  Guage  which  will  carry  50 
amperes  from  a  dynamo  to  lamps  which  are  500  feet  away,  the  drop 
to  be  about  10  volts.  .  Then,  M^21*5?*50^  52500.  A  No.  36.  &  S. 
wire  has  a  cross  section  of  52634  circular  mils  and  so  should  be 
used,  as  it  is  the  wire  of  nearest  siz*e. 

The  volts  drop  in  pressure  in  any  case  is  fixed  by  the  pressure  at 
the  dynamos  and  the  percentage  loss  of  pressure  which  may  be 
allowed.  For  instance,  if  the  pressure  at  the  dynamos  is  125  volts 
and  the  loss  is  tec  percent,  then  the  volts  drop  is  12.5.  Up  to  the 
present  time,  it  has  not  been  found  commercially  possible  to 
produce  incandescent  lamps  for  a  higher  pressure  than  about  115 
volts,  and,  consequently,  nearly  all  incandescent  lighting  with  contin- 
uous currents  is  done  at  a  pressure  between  100  and  115  volts  and 
each  1 6  candle-power  lamp  takes  about  one-half  an  ampere  of  cur- 
rent. If  by  any  means  the  pressure  at  the  lamps  could  be  doubled 
without  any  change  in  the  amount  of  light  given  out  for  each  100 
watts,  each  16  candle-power  lamp  would  require  only  about  one- 
fourth  of  an  ampere.  The  pressure  being  doubled,  the  number  of 
volts  in  a  given  percentage  loss  would  also  be  doubled.  We  see, 
therefore,  that  the  current  divided  by  the  drop  is  only  one-fourth  as 
great  with  the  double  pressure,  so  that  the  wires  required  to  carry 
current  a  fixed  distance  for  200  volt  lamps  need  only  be  one-fourth  as 
heavy  as  those  required  to  carry  current  for  the  same  number  of 100 


185 


FIG.  211. 

volt  lamps,  or  putting  the  statement  in  another  way,  the  weight  of 
copper  which  is  required  to  supply  current  at  a  fixed  percentage  loss 
of  pressure  to  a  number  of  100  volt  lamps  at  a  certain  distance  from 
the  dynamo  will  serve  to  supply  four  times  as  many  lamps  of  double 
the  pressure.  In  the  same  way,  it  may  be  seen  that  if  the  pressure  is 
increased  from  100  volts  to  300  volts,  the  wires  required  to  convey  a 
given  supply  of  power  a  certain  distance  may  be  reduced  to  one-ninth 
as  great  a  cross-section,  and  therefore  to  one-ninth  the  weight,  of 
those  required  for  the  100  volt  distribution. 

The  general  rule  may  be  given  as  follows:  When  a  given  amount 
of  power  is  transmitted  by  electricity  over  a  certain  distance  at  afixea 
percentage  loss,  the  cross-section  of  the  wires,  and  therefore  their  weight, 
is  in  inverse  proportion  to  the  square  of  the  pressure.  This  rule  applies 
equally  whether  the  current  is  used  for  producing  light,  operating  sta- 
tionary motors  or  running  street  cars;  the  transmission  of  electricity 
along  a  wire  from  a  dynamo  at  one  point  to  be  used  at  another  point 
is  electric  transmission  of  power,  whatever  may  be  the  purposes  for 
which  the  current  is  used.  Electric  lamps,  arc  or  incandescent,  may  be 
operated  on  the  same  circuits  with  electric  motors  or  electric  heaters, 
or  electric  lamps  may  be  taken  out  of  a  circuit  and  electric  motors 
put  in  their  place,  or  vice  versa,  without  altering  the  conditions.  It 
is  well  known  that  in  many  cities  electric  arc  and  incandescent 
lamps,  stationary  motors  and  electrically  heated  flatirons  and  curling 
irons  are  all  furnished  with  the  power  necessary  for  their  operation 
from  the  same  circuits.  Electric  street  cars  are  often  furnished  with 
light,  heat  and  power,  from  the  current  conveyed  to  the  car  by  the 
trolley  wire. 

It  is  very  easy  to  make  the  pressure  quite  high  in  circuits  which 
are  arranged  to  transmit  power  from  a  central  station  to  electric 
motors  alone,  and  thus  keep  the  weight  of  the  wires  required  within  a 
reasonable  limit,  since  the  electric  motors  may  have  their  windings 
designed  for  any  reasonable  pressure.  Five  hundred  volts  is  quite 
commonly  used  for  circuits  which  are  specially  intended  to  supply 
current  to  stationary  motors  and  street  car  motors.  Incandescent 
lamps  may  be  used  on  the  circuits,  but  it  is  necessary  to  use  them  in 
sets  of  five  100  volt  lamps  connected  in  series  (Fig.  211.)  This  is 
the  arrangement  which  is  used  for  lighting  electric  cars.  For  gen- 


186 


0- 
•0 

o- 


KH 

•o 

0 

o 


FIG.  212. 

eral  purposes,  such  an  arrangement  is  not  at  all  satisfactory,  because 
all  the  lights  of  each  set  must  either  burn  or  be  extinguished.  It  is 
not  possible  to  have  only  one  or  two  lamps  of  a  set  burn  at  once. 

Putting  only  two  100  volt  lamps  in  series  on  a  200  volt  circuit  is 
more  satisfactory  than  having  five  lamps  in  a  set,  but  even  this  is 
not  satisfactory  for  general  use.  By  means  of  the  three  wire  system, 
nearly  as  much  saving  in  copper  may  be  effected  as  by  doubling  the 
pressure,  and  yet  the  individual  lamps  are  entirely  independent.  A 
diagram  of  the  arrangement  of  the  three  wire  system  is  shown  in  Fig. 
212.  A  and  B  are  two  dynamos:  the  positive  terminal  of  the  first  is 
connected  to  the  positive  line  wire,  its  negative  terminal  is  connected 
to  the  positive  terminal  of  the  second,  and  the  negative  terminal  of 
the  second  is  connected  to  the  negative  line  wire.  A  third  wire 
called  the  neutral  wire  is  connected  at  a  point  between  the  two 
dynamos  and  runs  out  on  the  line  with  the  positive  and  negative 
wires.  The  electric  lamps  are  connected  between  the  positive  and 
neutral  wires  and  the  negative  and  neutral  wires,  the  lamps  being 
arranged  so  that  the  number  on  each  side  of  the  system  is  as  nearly 
equal  as  possible  and  so  that  the  number  of  lamps  likely  to  burn  at  one 
time  is  equal  on  the  two  sides.  When  this  condition  is  fulfilled  as  shown 
in  the  figure,  the  system  is  said  to  be  balanced,  and  the  current  flows 
from  the  positive  pole  of  the  first  dynamo  through  the  positive  wire  to 
the  lamps  on  the  positive  side,  through  these  lamps  to  the  neutral  wire 
and  thence  directly  through  lamps  on  the  negative  side  to  the  nega- 
tive wire,  and  it  returns  to  the  negative  terminal  -of  the  second 
dynamo.  If  the  system  is  balanced,  no  current  returns  to  the  dynamos 
through  the  neutral  wire,  and  the  dynamos  operate  exactly  as 
though  they  were  simply  connected  in  series.  The  function  of  the 
neutral  wire  is  to  distribute  the  current  from  the  lamps  on  the  posi- 
itive  side  of  the  system  to  those  on  the  neutral  side,  and  if  more  lamps 
are  in  use  on  one  side  of  the  system  than  on  the  other,  the  extra  cur- 
rent is  deli vered  or  returned  to  the  dynamos  through  the  neutral  wire. 
Fig.  213  shows  by  the  arrows  the  way  in  which  the  current  is  distributed 
to  the  lamps  on  a  balanced  system  in  which  the  lamps  are  not  exactly 
opposite  to  each  other.  Fig.  214  is  a  diagram  of  a  three  wire  sys- 


287 


OF  THE 
TJNIVERSI 

OF 


hOH 

-o 

0 


-o- 

•0 

k>J 


•0- 
-0- 

•o 


-0- 
-0 

o- 

-0 


FIG.  218. 


FIG.  214. 


FIG.  215. 

tern  with  more  lights  connected  to  the  positive  than  to  the  negative 
side  of  the  system.  The  arrows  show  the  direction  in  which  the 
current  flows  in  the  wires.  The  positive  wire  carries  enough  current 
to  supply  the  lamps  on  the  positive  side  of  the  system,  and  the  dif- 
ference between  the  current  required  to  supply  the  two  sides  returns 
through  the  neutral  wire.  The  positive  dynamo  therefore  carries 
more  load  than  the  negative  dynamo.  In  Fig.  215  the  dynamos  of 
Fig.  214  are  replaced  by  pumps  and  the  lamps  by  water  motors. 
Again  the  arrows  show  the  direction  of  the  streams  in  the  system  of 
piping. 


18S 


FIG.  216. 

The  plan  for  increasing  the  dynamo  pressure  used  to  supply  in- 
candescent lamps,  by  connecting  the  lamps  practically  in  series  and 
yet  making  them  really  independent  of  each  other  by  means  of  a 
neutral  wire,  may  be  extended.  Fig. '  2 16  is  a  diagram  of  the  arrange- 
ment with  four  lamps  in  series  and  five  wires.  This  is  known  as  the 
five  wire  system. 

The  weight  of  wire  required  in  a  three  wire  system  amounts  to 
a  little  more  than  one-fourth  of  the  weight  required  for  a  two  wire 
system  because  of  the  introduction  of  the  neutral  wire.  The  actual 
weight  required  is  about  three-eighths  of  that  in  a  two  wire  system. 
This  saving  in  the  weight  of  copper  is  a  very  important  factor  to 
large  electric  lighting  companies,  as  their  copper  feeders  and  mains 
cost  a  great  deal  of  money.  The  saving  by  the  five  wire  system  is 
proportionally  greater  than  that  effected  by  the  three  wire  system, 
but  it  causes  greater  difficulty  in  keeping  the  pressure  perfectly  con- 
stant at  the  lamps.  The  three  wire  system  is  used  in  a  great  many 
plants  in  this  and  foreign  countries  which  have  been  constructed  by 
the  Edison  Co.  All  the  large  Bdison  illuminating  plants  in  large 
American  cities  use  the  three  wire  system.  The  five  wire  system  is 
constructed  by  the  Siemens  &  Halske  Co.  It  is  used  in  a  large  plant 
in  Berlin  and  elsewhere. 

Electric  motors  are  usually  operated  on  two  wires  except  when 
they  are  connected  to  electric  lighting  circuits,  because,  as  already 
explained,  they  may  be  wound  for  any  desired  pressure  and  it  is  not 
necessary  to  use  low  pressure  motors  connected  in  series  in  order  to 
get  an  economical  pressure  for  distribution. 

Since  the  armatures  of  electric  motors  are  usually  of  low  resist- 
ance, it  is  necessary  to  connect  a  resistance  box  in  series  with  the 
armature  when  starting  a  motor  on  a  constant  pressure  circuit.  This 
resistance  box  contains  sufficient  resistance  so  that  a  little  more 
than  the  ordinary  full  load  current  is  allowed  to  pass  through  the 
armature  when  it  is  standing  still.  The  machine  consequently  starts 


O.P.  CUfrOUT  BOX 


.COMMUTATOR. 

FIG.  217. 

easily.  As  the  armature  speeds  up,  its  counter  electric  pressure 
grows,  and  the  resistance  in  series  with  the  armature  may  then  be 
slowly  reduced  and  finally  be  cut  out  altogether.  Fig.  217  shows 
the  connections  to  constant  pressure  mains  of  a  shunt  motor  with  its 
starting  box. 

Copyrighted,  1894, 


The  National  School  of  Electricity. 

REVIEW  OF  LESSON  XXII. 

Points  for  Review:     1.     What  is  the  principle  of  the  incandescent  lamp? 

2.  Why  do  not  the  carbons  of  incandescent  lamps  quickly  burn  away? 

3.  How  is  the  vacuum  of  incandescent  lamps  produced? 

4.  Of  what  materials  are  incandescent  lamp  filaments  ordinarily  made? 

5.  How  is  the  original  material  of  the  filament  converted  into  carbon? 

6.  Why  are  incandescent  lamps  better  than  arc  lamps  for  general  indoor  lighting? 

7.  Upon  what  kind  of  circuits  are  incandescent  lamps  and  motors  usually  operated? 

8.  Why  is  a  constant  pressure  necessary  for  the  operation  of  incandescent  lamps 
connected  in  parallel? 

9.  What  is  the  effect  on  an  incandescent  lamp  of    "over-running"  it — that  is,  of 
running  it  at  too  high  a  pressure? 

10.  Why  is  there  always  a  loss  of  pressure  in  the  wires  which  convey  current  from 
dynamo  to  lamps? 

11.  What  is  meant  by  circular  mils? 

12.  Suppose  it  is  desired  to  convey  150  amperes  from  a  dynamo  to  lamps  which  are 
800  feet  away,  and  the  drop  is  not  to  exceed  25  volts;  what  must  the  cross-section  of  the 
wires  be?     To  what  number  in  the  B.  &  S.  gauge  does  this  most  nearly  correspond? 

13.  If  the  pressure  at  the  dynamos  is  130  volts,  and  the  loss  is  20  per  cent,  what  is 
the  pressure  at  the  lamps?     What  is  the  pressure  at  the  lamps  if  the  loss  is  10  per  cent? 

14.  If  the  pressure  desired  at  the  lamps  is  104  volts  and  the  drop  on  the  line  is  20 
per  cent  of  the  dynamo  pressure,  what  must  the  pressure  be  at  the  dynamos? 

15.  How  does  the  weight  of  copper  required  in  transmitting  a  given  amount  of  power 
over  a  given  distance,  at  a  fixed  percentage  loss,  vary  with  the  pressure  at  the  dynamos? 

16.  What  is  meant  by  the  3-wire  system? 

17.  Why  is  the  3-wire  system  used  in  electric  light  plants? 

18.  Why  are  starting  resistances  used  with  electric  motors  which  are  operated  on 
constant  pressure  circuits? 

LBSSON  XXIII. 

CONSTRUCTION     OF     ELECTRIC    LIGHT  ,  AND 
POWER  CIRCUITS  AND  THEIR  TESTING. 

The  wires  of  nearly  all  electric  light  and  power  plants  which 
are  not  located  in  large  cities  are  carried  on  wooden  poles.  The  con- 
struction of  the  pole  lines  is  quite  similar  to  that  of  telegraph  and 
telephone  lines  explained  in  L,essqn  XVII,  but  as  a  general  rule 
electric  light  and  power  lines  carry  fewer  but  much  heavier  wires. 
The  sizes  of  the  wires  depend  upon  the  current  transmitted  over 
them,  their  length,  and  the  drop  of  pressure  which  is  permitted  to 
occur  in  them,  and  are  determined  by  the  method  explained  in  the 
previous  lesson.  The  wires  are  always  of  copper  of  the  highest  ob- 
tainable conductivity,  and  they  ordinarily  vary  in  size  from  No.  10 
to  No.  oooo  B.  ,&  S.  gauge,  or  from  about  TV  of  an  inch  in  diameter 
to  nearly  y2  of  an  inch  in  diameter.  The  former  is  the  smallest 
wire  of  soft  copper  which  can  b^  depended  upon  not  to  break  from 
mechanical  strains  caused  by  the  wire  swaying  in  the  wind,  other 

191 


wires  falling  upon  it,  etc.;  while  the  latter  is  the  largest  solid  wire 
which  can  be  conveniently  handled.  Where  a  number  of  large  wires 
are  run  on  the  same  pole  line,  extra  heavy  poles  and  cross  arms  are 
used.  The  glass  insulators  which  are  used  for  electric  light  and 
power  lines  are  rather  heavier  than  those  illustrated  in  Lesson  XV II, 
which  are  used  for  telegraph  and  telephone  lines,  and  have  a  deeper 
groove  (Fig.  219).  The  groove  in  these  insulators  is  so  large  com- 
pared with  that  in  other  insulators  that  they  are  commonly  called 
deep  groove  insulators. 

The  wires  used  upon  overhead  telegraph  and  telephone  lines  are 
not  covered  with  insulation  and  the  same  is  true  of  low  pressure 
electric  light  lines.  For  instance,  the  overhead  wires  used  to  dis- 
tribute current  for  incandescent  lighting  by  the  ordinary  3-wire 
system  are  almost  always  bare,  and  the  glass  bells  at  the  point  of 
support  are  depended  on  to  give  a  satisfactory  insulation.  This  is 
perfectly  safe  when  the  pressure  is  as  low  as  in  the  ordinary  3-wire 
system,  where  the  pressure  between  the  positive  and  negative  wires 
is  seldom  higher  than  260  volts.  When  the  pressure  used  on  overhead 
lines  is  higher  than  300  volts,  it  is  usual  to  use  insulated  wires.  The 
insulation  consists  of  a  continuous  braided  cotton  covering  of  two  or 
three  thicknesses,  which  is  thoroughly  soaked  in  some  insulating  com- 
pound. As  the  insulation  is  supposed  to  be  partially  waterproof,  such 
wire  is  often  called  weather-proof 'wire.  The  insulating  compound 
which  is  used  is  almost  always  black.  Black  weather-proof  wire  is  used 
for  the  overhead  lines  of  power  plants  which  distribute  current  to 
moto;s  at  a  pressure  of  500  volts,  for  the  overhead  lines  of  alternating 
current  electric  light  plants  which  use  a  pressure  of  1,000  volts,  for  the 
feeders  of  electric  railway  plants,  arc  light  wires,  etc.  It  has  become 
an  almost  universal  custom  in  this  country  to  use  No.  6  B.  &  S. 
gauge  weather-proof  wires  for  arc  light  lines.  As  the  arc  current 
seldom  exceeds  10  amperes,  the  loss  of  pressure  in  a  No.  6  wire 
several  miles  in  length  is  not  very  great,- and  it  is  a  convenient  and 
economical  size  to  use. 

The  circuits  for  electric  lighting  and  power  are  always  complete 
wire  circuits,  as  the  use  of  the  ground  for  returning  large  currents  is 
likely  to  cause  difficulties  from  the  uncertain  resistance  of  ground 
plates,  and  &g>ounded  electric  light  circuit  always  introduces  a  risk 
of  fire  in  each  house  that  it  enters.  For  the  latter  reason  fire  insur- 
ance men  or  Underwriters  refuse  to  approve  the  use  of  a  ground 
return  for  the  distribution  of  electric  light  and  power  where  the  wires 
enter  buildings  insured  by  them. 

In  the  large  cities,  electric  light  wires  are  put  underground.  In 
this  case,  two  entirely  different  systems  may  be  used.  The  first  is 
like  that  described  in  Lesson  XVII,  and  is  often  called  the  drawing  in 
system,  because  the  lead  covered  cables  are  pulled  or  drawn  into  con- 
duits from  manhole  to  manhole.  Electric  light  cables  differ  very  much 


192 


from  telephone  and  telegraph  cables,  as  they  usually  contain  only 
one  wire,  and  seldom  contain  more  than  two  wires.  Fig.  220  shows  a 
single  conductor  electric  light  cable  and  a  two  conductor  or  duplex 
cable.  The  electrostatic  capacity  of  electric  light  cables  is  not  a 
matter  of  great  importance,  and  the  choice  of  insulating  material  for 
such  cables  may  therefore  depend  almost  wholly  on  mechanical  and 
insulating  qualities.  In  some  cables,  rubber  compounds  are  used  foi 
the  insulating  material.  In  this  case,  the  copper  conductor  is 
covered  with  a  layer  of  rubber  compound,  and  over  this  is  pressed  the 
lead  sheathing.  The  thickness  of  the  rubber  insulation  depends,  to 
some  extent,  upon  the  electrical  pressure  at  which  current  is  trans- 
mitted through  the  conductors,  but  it  is  usually  between  y&  and  J^  of 
an  inch,  while  the  thickness  of  the  lead  covering  is  sufficient  to  give 
a  satisfactory  protection  to  the  insulation  against  mechanical  injury 
and  to  protect  the  insulation  from  contact  with  moisture  or  harmful 
gases. 

The  insulation  of  some  cables  is  made  by  closely  wrapping  the 
conductor  with  strips  of  paper  which  have  been  soaked  in  an  insulat- 
ing compound  so  as  to  make  it  quite  soft  and  flexible.  The  thick- 
ness of  this  paper  wrapping  is  made  about  the  same  as  that  of  rubber, 
and  a  lead  sheathing  is  put  on  in  exactly  the  same  manner  as  on  the 
rubber  insulated  cables.  A  third  style  of  insulation  consists  of  a 
thick  braiding  or  wrapping  made  up  of  several  layers  of  cotton  or  jute 
which  is  soaked  in  an  insulating  compound  quite  similar  to  that  used 
for  weather-proof  wires.  This  is  also  covered  with  a  lead  sheathing. 
•  The  latter  cables  are  often  said  to  have  fibrous  insulations  on 
account  of  the  character  of  the  materials  used.  As  fibrous  material 
will  rapidly  absorb  moisture  and  its  insulating  qualities  thus  become 
ruined,  it  is  absolutely  necessary  that  the  lead  sheathing  shall  con- 
tain no  holes,  however  small,  and  the  ends  of  the  cables  must  be 
protected  from  moisture  with  extreme  care.  The  protection  of  rubber 
insulation  from  moisture  is  not  so  important,  but  moisture  may  even 
here  have  a  serious  effect,  so  that  the  most  careful  inspection  and 
handling  of  the  cables  is  advisable. 

The  second  method  of  laying  underground  conductors  for  the 
distribution  of  electric  current  is  often  called  the  solid  or  built  in  sys- 
tem, because  the  insulated  conductors  with  their  protecting  conduit 
are  laid  in  the  ground  together.  In  this  case,  if  any  harm  comes  to 
either  the  conductor  or  its  insulation,  the  street  must  be  dug  up  at 
the  place  of  "trouble"  before  repairs  can  be  made.  With  the  "draw- 
ing in"  system,  repairs  maybe  made  by  simply  pulling  out  that  sec- 
tion of  cable  between  two  manholes  which  contains  the  injury,  and 
replacing  it  with  a  piece  of  good  cable. 

The  "built  in"  system  is  commonly  used  for  low  pressure  dis- 
tribution of  electric  current,  and  for  this  purpose  gives  excellent  sat- 
isfaction. Nearly  all  the  great  electric  illuminating  companies  in 


193 


our  large  cities  which  use  the  3-wire  system  have  their  conductors 
laid  in  this  manner.  For  high  pressure  distribution,  the  "built  in" 
system  of  underground  conductors  is  not  as  satisfactory  as  the  "draw- 
ing in"  system. 

The  most  commonly  used  arrangement  of  the  "built  in"  system 
is  that  known  as  "Edison  tubing."  This  was  introduced  about  a 
dozen  years  ago,  and  was  used  in  its  original  form  in  the  laying  of 
the  conductors  connected  with  the  old  Pearl  street  central  station  in 
New  York  city,  the  first  great  central  station  for  the  general  distri- 
bution of  the  electric  current.  Edison  tubing  was  the  earliest,  and  for 
many  years,  the  only  scheme,  in  which  the  details  of  a  general 
underground  system  for  distributing  electric  current  were  satisfac- 
torily worked  out. 

On  account  of  the  experience  gained  in  laying  the  conductors 
for  the  various  large  Edison  electric  illuminating  companies,  the 
system  of  tubing  has  been  considerably  changed  since  its  first  intro- 
duction. As  the  tubes  are  now  made,  they  usually  contain  three 
copper  rods — the  positive,  negative  and  neutral  conductors  of  the 
3-wire  system.  These  rods,  which  are  somewhat  over  20  feet  long,  are 
each  wound  with  a  spiral  of  manilla  rope,  and  are  then  laid  side  by 
side  but  separated  from  each  other  by  the  ropes.  Another  spiral  of 
rope  is  wound  around  the  bunch  to  hold  the  conductors  firmly 
together.  The  bunch  of  three  conductors  is  placed  in  an  iron  pipe 
twenty  feet  long,  in  such  a  way  that  the  copper  rods  stick  out  a  few 
inches  at  each  end.  One  end  of  the  pipe  or  tube  is  then  connected  to  a 
pump  by  means  of  which  a  vacuum  is  created  in  the  tube,  and,  finally, 
hot  black  insulating  compound  is  pumped  into  the  tube  until  all  the 
open  space  inside  of  it  is  filled.  The  insulating  compound  is  of  a 
bituminous  nature  and  hardens  when  it  is  permitted  to  cool.  Fig. 
221  shows  a  cross-section  of  a  "tube"  in  which  AAA  are  the  copper 
conductors,  C  is  the  iron  pipe,  and  B  is  the  insulating  compound. 
Fig.  222  is  a  complete  length  of  the  completed  tubing,  showing  the 
form  in  which  it  is  delivered  from  the  factory  to  be  laid  in  the 
ground.  For  the  purpose  of  laying  the  tubes  a  trench  is  dug,  and 
the  20  feet  lengths  are  laid  down  end  to  end.  The  conductors 
in  successive  tubes  are  joined  by  means  of  flexible  copper  connectors 
(Fig.  223)  having  solid  copper  heads  with  holes  which  slip  over  the 
ends  of  the  rods  where  they  are  soldered  fast.  Ball-like  caps  are 
bolted  fast  to  the  tube  ends  and  over  these  is  bolted  a  split  coupling 
box  which  covers  the  joint  (Fig.  224).  Only  one-half  of  the  coup- 
ling box  is  shown  in  the  figure.  In  the  top  of  this  coupling  box  is 
a  hole  through  which  hot  insulating  compound  may  be  poured  when 
the  joint  is  completed,  and  the  hole  is  then  covered  with  an  iron  cap. 

The  arrangement  here  described  is  very  satisfactory  since  it 
offers  an  electric  company  the  same  ease  as  a  gas  company  or  a  water 
company  in  making  connections  to  houses.  A  branch  to  a  house,  or 


U      ) 


FIG.  219. 


FIG.   224. 


195 


service  connection,  as  it  is  called,  may  be  connected  to  the  main  con- 
ductors at  any  coupling  box  by  simply  changing  the  plain  box  to  a 
T  box  (Fig.  225). 

Several  different  arrangements  of  "built  in"  conductors  have 
been  used  in  England,  France  and  Germany.  One  of  these  consists 
of  a  simple  brick,  concrete  or  cast-iron  trench,  or  culvert,  in  which  the 
copper  rods  or  bars  used  for  conductors  are  placed  on  porcelain  insula- 
tors. Fig.  226  shows  an  end  view  and  a  side  view  of  such  a  culvert  at  a 
point  where  a  set  of  insulators  is  located.  One  of  the  most  remarkable 
arrangements  of  the  ubuilt  in"  system  is  that  used  in  London  to 
distribute  electric  current  by  the  two  wire  system  from  the  noted 
Deptford  central  station.  The  conductors  in  this  case  are  enclosed 
in  an  iron  pipe,  as  are  the  conductors  in  the  Edison  system,  but  the 
conductors  themselves  are  copper  tubes  placed  one  inside  of  the 
other  instead  of  being  rods  placed  side  by  side.  The  space  between 
the  conductors  is  filled  with  insulation  which  consists  of  brown 
paper  soaked  in  an  insulating  compound  (Fig.  227).  The  same 
kind  of  insulation  is  also  placed  between  the  outer  conductor  and 
the  iron  protecting  pipe.  This  conducting  system  was  designed  and 
laid  down  to  transmit  current  at  the  enormous  and  unusual  pressure 
of  10,000  volts,  and  it  has  served  its  purpose  very  well.  As  the 
tubes  could  not  be  made  in  lengths  much  greater  than  20  feet,  joint- 
ing the  lengths  together  was  a  matter  of  much  difficulty  on  account 
of  the  concentric  arrangement  of  the  conductors. 

In  order  that  the  electrical  pressure  may  be  kept  the  same 
at  all  points  on  a  system  of  conductors  which  cover  a  large  district, 
the  conductors  must  be  divided  into  feeders  and  mains.  The  mains 
consist  of  the  conductors  to  which  lamps  or  motors  are  directly 
connected.  These  are  carried  all  through  the  streets  of  the  district  to 
which  current  is  to  be  supplied  and  are  often  joined  into  a  network  by 
means  of  fuses  located  in  manholes  or  junction  boxes  at  street  corners. 
The  current  is  supplied  to  the  mains  at  certain  central  points  called 
feeding  points  by  means  of  feeders  which  run  directly  to  the  feeding 
points  from  the  central  station  where  the  current  is  generated.  Fig.  228 
is  a  diagram  representing  the  arrangement  of  feeders  and  mains.  The 
points  marked  i,  2,  3,  4,  are  the  feeding  points,  and  A,  B,  C,  D,  are 
houses  to  which  current  is  supplied  through  service  connections. 
The  figure  shows  three  wires  in  each  main  and  feeder,  as  is  required 
in  a  3-wire  system.  By  carefully  calculating  the  resistance  of  each 
feeder  and  main  before  the  system  is  constructed  it  is  possible  to  get 
a  very  uniform  pressure  over  the  whole  distributing  system.  In 
order  that  the  dynamo-men  may  regulate  the  pressure  of  the  dyna- 
mos in  the  central  station  so  as  to  keep  the  pressure  uniform  at  the 
feeding  points,  it  is  necessary  to  have  voltmeters,  or  pressure  indi- 
cators, in  the  dynamo  room,  which  show  the  pressure  at  the  feeding 
points.  For  this  purpose  wires  called  pressure  wires  are  run  from 


190 


FIG.  225. 


$&?&tt&X&ttSm 


FIG.  226. 


FIG.  228. 


197 


S^Z*     OF  THE 

(-0NIVERS: 

\^c> 


the  feeding  points  to  the  voltmeters  in  the  dynamo  room.  A  some- 
what similar  network  of  pipes  is  sometimes  used  in  the  distribu- 
tion of  gas  and  water  in  large  cities. 

The  importance  of  that  part  of  electric  lighting  circuits  which 
is  inside  of  buildings  cannot  be  overestimated.  A  central  station  may 
be  built  upon  the  best  plan  to  supply  current  through  a  perfect  dis- 
tributing system,  but  a  safe  and  satisfactory  light  will  not  be  given 
if  the  inside  wiring  is  poorly  planned  and  put  in  place.  Fires  which 
occur  on  account  of  the  electric  light  wires  in  houses  are  always  caused 
either  by  the  use  of  poor  material,  careless  planning,  or  bad  work- 
manship when  the  inside  wiring  was  put  in,  and  if  the  wiring  is  done 
properly  it  is  almost  impossible  for  Jires  to  be  caused  by  an  electric 
lighting  system.  On  the  other  hand,  poorly  constructed  wiring  is  a 
constant  danger  aud  should  not  be  permitted  anywhere.  On  account 
of  the  danger  which  may  be  caused  by  unscrupulous  .or  untrustworthy 
wiremen,  it  is  usual  in  large  cities  to  have  official  inspectors  to  exam- 
ine and  test  all  electric  light  work  placed  within  buildings.  It  is  the 
duty  of  these  inspectors  to  see|  that  the  work  is  safely  and  properly 
done  in  accordance  with  rules  fixed  by  the  city  authorities  and 
approved  by  the  fire  underwriters.  Even  with  such  inspection  the 
work  is  not  always  done  in  the  best  manner,  yet  comparatively  few 
important  fires  have  been  caused  by  electric  wires  and  a  great 
majority  of  the  accidents  laid  to  the  door  of  electricity  are  due  to  some 
other  cause. 

For  ordinary  wiring  inside  of  a  building  only  the  very  best  rubber 
covered  wire  should  be  used.  A  great  many  factories  produce  rubber 
covered  wire  for  use  in  inside  wiring  and  much  of  it  is  very  poor, 
so  that  great  care  is  necessary  in  selecting  material. 

The  wires  may  be  run  in  buildings  according  to  three  entirely 
different  methods.  In  the  first,  the  wires  are  run  upon  the  surface  of 
ceilings  or  walls  in  plain  sight  and  are  held  in  place  by  means  of 
cleats  made  of  wood  or  porcelain  (Fig.  229).  This  is  the  commonest 
arrangement  of  wiring  in  stores  and  other  buildings  where  the 
position  of  the  wires  in  plain  sight  is  not  objectionable.  As  the  wires 
are  in  plain  sight  and  therefore  can  be  easily  inspected  at  all  times,  open 
work  or  cleat  work,  as  this  arrangement  is  called,  is  a  safe  and  satis- 
factory arrangement  of  the  wiring,  provided  the  wires  and  appliances 
are  all  out  of  reach  so  that  they  cannot  be  tampered  with.  In  damp 
places  or  in  places  where  .there  are  fumes  which  attack  the  insulation, 
the  wires  are  often  supported  on  porcelain  knobs  (Fig.  125,  Lesson 
XVII),  instead  of  being  held  against  the  walls  by  cleats,  as  an  addi- 
tional safeguard. 

There  are  many  places  where  the  appearance  of  open  work  is 
objected  to,  but  where  the  wires  may  be  placed  in  wooden  casings  or 
mouldings  which  are  fastened  to  ceilings  or  walls  in  plain  sight.  This 
is  an  exceedingly  safe  and  satisfactory  arrangement,  since  the  wires 


198 


r7  nJ! 


© 


FIG.  229. 


FIG.  231. 


FIG.  238. 


o 


O 

O 


FIG.  232. 


FIG.  230. 


FIG.  233. 


199 


are  well  protected  from  mechanical  injury  or  from  being  tampered 
with,  and  yet  the  condition  of  the  wiring  may  be  easily  seen  at  any 
time  by  a  simple  inspection.  Common  forms  of  mouldings  are  shown 
in  Fig.  230.  These  may  be  made  of  any  desired  wood,  though  pine 
is  most  commonly  used. 

In  the  third  method  of  running  wires,  they  are  placed  entirely 
out  of  sight,  or  concealed.  This  may  be  done  in  various  ways;  the 
commonest  and  at  the  same  time  the  least  safe  and  satisfactory  way 
is  to  fasten  the  wires  to  the  ceilings  and  walls  of  the  building  before 
the  plastering  is  put  on.  The  wires  are  then  entirely  covered  by  the 
plaster,  so  that  it  is  impossible  to  examine  or  repair  them  without 
injury  to.  the  walls,  and,  indeed,  the  position  of  the  wires  in  the  walls 
•is  often  forgotten  in  a  few  months  after  the  building  is  finished,  so 
that  repairs  are  doubly  difficult  to  make.  This  arrangement  of  the 
wires  is  made  more  unsafe  because  the  plaster  upon  the  walls  often 
spoils  the  insulating  qualities  of  the  rubber  coverings  and  the  wires 
become  grounded  as  a  consequence. 

In  buildings  with  wooden  floors  and  partitions  the  wires  are 
often  fastened  to  the  floor  joists  or  partition  studding  by  means  of  cleats 
or  porcelain  knobs.  When  this  is  properly  done,  the  wires  are 
not  likely  to  be  injured  by  plaster  or  dampness,  but  the  disadvantage 
that  they  cannot  be  examined  is  still  present.  They  are  also  liable 
to  injury  by  plumbers,  carpenters  or  other  workmen  who  are  engaged 
in  making  repairs  or  alterations  to  the  building. 

When  it  is  necessary  to  conceal  electric  light  wires  it  is  much 
better  to  arrange  a  hidden  moulding  or  conduit  to  contain  them. 
This  may  be  hidden  behind  decorations  or  other  objects  on  the  walls 
or  may  be  laid  neatly  under  the  floors  or  the  plaster  (Fig.  231.) 
Special  tubes  are  made  to  be  used  as  conduits  for  inside  wiring. 
These  are  called  "interior  conduit,"  "vulca  duct,"  etc.  (Fig. 232), 
and  are  used  to  a  considerable  extent.  When  properly  used  they  are 
excellent,  but  are  no  better  than  any  strong,  watertight  insulating  tube, 
such  as  an  iron  or  brass  pipe  with  an  insulating  lining.  Interior  tubing 
made  of  insulating  material  was  originally  intended  to  take  the  place 
of  the  rubber  insulation  on  the  wires  so  that  they  could  be  used  with 
a  cheap  cotton  covering,  but  it  has  been  found  to  be  necessary  to  use 
the  best  rubber  insulation  on  wires  in  the  tubes  in  order  that  the 
wiring  may  give  satisfaction.  The  advantages  of  tubes  are  that  the 
wires  are  protected  from  mechanical  injury  and  from  contact  with 
plaster,  moisture,  etc. 

The  plan  of  the  wiring  in  a  building  depends  a  great  deal  on  the 
size  and  construction  of  the  building,  but  in  its  details  it  should 
always  fulfill,  not  only  in  the  letter  but  in  the  spirit,  the  require- 
ments of  the  Underwriters  which  are  laid  down  in  special  printed 
rules.  In  small  buildings  supplied  with  current  from  a  central  sta- 
tion, the  simplest  plan  for  concealed  wiring  is  what  may  be  called 


300 


FIG.  234. 

the  "distributing  system."  Heavy  service  wires  are  led  from  the 
street  mains  of  the  electric  light  company  through  a  fuse  block  or 
cut-out  (Fig.  233)  to  a  convenient  central  point  in  the  building.  At  this 
point  the  service  wires  terminate  in  a  number  of  fuse  blocks  from 
each  of  which  a  circuit  of  smaller  wire  runs  out  to  supply  a 
limited  number  of  lamps,  usually  between  5  and  15.  Fig.  234  shows 
such  a  plan  of  wiring  so  plainly  that  no  additional  description  is 
necessary.  In  the  figure,  s,  s,  s  are  switches  for  turning  the  lights  on 
and  off,  and  c  is  a  fuse  block  used  to  protect  a  small  branch  circuit 
which  for  convenience  is  connected  to  one  of  the  taps  instead  of 
being  run  back  to  the  distributing  center.  By  this  arrangement  of 
the  distribution  any  serious  trouble  which  occurs  on  one  branch  or 
tap  causes  the  fuses  at  the  distributing  center  which  belong  to 
the  branch,  to  melt.  This  disconnects  the  defective  branch  from  the 
service  wires  without  interfering  with  the  other  branches.  The 
location  of  all  the  fuse  blocks  at  a  central  point  makes  it  convenient 
to  replace  fuses,  and  the  fuse  blocks  can  be  so  protected  that  a  fire 
cannot  possibly  be  caused  by  the  arc  which  sometimes  occurs  when 
a  fuse  melts  or  blows. 


201 


FIG.  235. 

Another  plan  for  wiring  a  building  is  shown  in  Fig.  235.  In 
this  figure,  a  heavy  trunk  circuit  runs  from  the  main  cut-out  in  the 
cellar  to  the  top  of  the  house,  and  the  lamp  taps  branch  off  from 
the  trunk  at  each  floor;  s,  s,  s  are  switches  for  controlling  the  lights 
and  c,  c,  c  are  fuse  blocks.  This  plan  makes  it  necessary  to  scatter 
the  fuse  blocks  through  different  parts  of  the  house,  which  is  a  dis- 
advantage. 

In  large  buildings,  a  combination  of  the  two  plans  just  ex- 
plained is  used,  and  feeding  trunks,  or  feeders,  are  run  from  the  main 
fuse  block  to  several  distributing  centers  at  convenient  points  in  the 
building.  One  feeder  with  its  mains  is  shown  in  Fig.  236,  where 
XA  is  the  feeder  running  from  the  main  cut-out,  or  from  the  dynamo 
room  if  a  special  lighting  plant  is  located  in  the  building,  and  B,  B, 
D,  D  is  a  main  which  runs  down  and  up  so  as  to  supply  current  to 
the  different  floors  of  the  building.  Fuse  blocks  are  placed  at 
each  rectangle  to  protect  the  parts  of  the  circuit  beyond  it.  The 
horizontal  lines  are  mains  which  run  along  each  floor  to  carry  current 
to  the  distributing  centers  which  are  shown  by  the  rectangles  at  Y, 
Y1,  Y2,  Y3.  The  lamp  taps  which  are  run  from  the  centers  to  the 
lamps  are  represented  by  the  short  spiral  lines.  Only  one  line  is 
used  in  this  figure  to  represent  the  circuit,  which  may  be  either  2- 
wire  or  vwire. 


KG" ^ Qy7 


FIG.  236. 

For  very  large  buildings,  the  plan  shown  in  Fig.  236  may  be 
extended  by  running  feeders  to  various  points  in  the  building,  from 
which  points  mains  run  to  the  distributing  centers.  The  feeding  points 
are  then  usually  joined  together  by  a  heavy  connecting  circuit  often 
called  a  crib.  This  is  shown  in  Fig.  237  where  E1,  E2,  E3,  E4  are 
feeders  running  to  four  feeding  points  in  a  building  which  are 
marked  K,  K,  K,  K.  These  points  are  joined  together  by  the 
crib  from  which  the  mains  run  off  to  the  various  centers  of  distri- 
bution. 

The  wiring  plan  in  a  large  building  is  seen  to  be  quite  similar 
to  the  plan  of  the  feeders  and  mains  used  in  distributing  electric 
current  from  a  central  station.  The  object  to  be  aimed  at  in  arrang- 
ing the  wires  in  either  case  is  to  keep  all  the  lamps  which  are  burn- 
ing at  one  time  as  nearly  as  possible  at  the  same  pressure,  and  also 
to  make  it  possible  to  keep  the  pressure  constant  regardless  of  the 
number  of  lamps  burning.  The  size  of  wires  used  at  any  place 
must  be  calculated  from  the  amount  of  current  which  the  wires 
carry  and  the  volts  drop  in  pressure  which  is  allowed.  The  calcu- 
lation sometimes  indicates  a  wire  which  is  too  small  for  safety,  and 
a  wire  smaller  than  No.  16  B.  &  S.  gauge  should  never  be  used  in 
inside  electric  light  wiring ;  neither  should  the  current  passed 
through  a  wire  exceed  the  "safe  carrying  capacity''  given  in 
Lesson  XXXII.  "Wiring  tables,"  which  give  the  sizes  of  inside 


203 


Jp* 


£'  E*  WE* 

FIG.  237. 

wires  required  to  supply  current  to  lamps  at  various  distances  from 
the  main  cut-outs,  are  to  be  found  in  many  trade  catalogues. 

A  great  many  details  relating  to  inside  wiring  can  only  be 
learned  by  observing  wiring  which  has  been  completed  in  a  proper 
manner,  but  a  great  deal  of  useful  information  relating  to  the  inci- 
dental material  can  be  obtained  from  the  catalogues  of  the  com- 
panies who  supply  electrical  material.  The  most  important  in- 
cidental material  is  fuse  blocks,  fuses,  switches  and  sockets. 
Fuse  blocks  now  invariably  consist  of  porcelain  bases  of  various 
forms  upon  which  are  carried  terminal  screws  for  the  con- 
nection of  the  fuses  and  the  circuit  wires.  Electric  light  fuses 
are  strips  or  wires  of  a  metal  alloy  which  melts  at  a  temperature  that 
is  so  low  that  the  melted  metal  cannot  possibly  cause  harm.  The  alloy 
is  usually  made  largely  of  lead  and  tin,  but  varies  a  great  deal.  The 
object  of  the  fuse  is  to  protect  the  wires  beyond  it  from  becoming 
overheated  through  some  accident.  The  size  of  the  fuse  at  any  point 
is  such  that  if  anything  occurs  to  cause  an  unsafe  current  to  flow 
through  the  wires  protected  by  it,  the  fuse  will  melt  and  cut  the  wires 
out  of  the  circuit.  Fuses  of  large  carrying  capacity  usually  have  ter- 
minals made  of  copper  (Fig.  238)  so  that  a  more  substantial  contact 
may  be  made  with  the  fuse  block  terminals.  Switches  and  sockets 
have  already  been  illustrated.  A  most  important  factor  in  locating  the 
position  of  centers  of  distribution  in  a  building  is  the  location  of  the 
lights.  The  next  lesson  will  enter  into  this. 

Copyrighted,  1894, 

2O4, 


The  National  School  of  Electricity. 

REVIEW  OF   LESSON  XXIII. 

Points  for  review.     1.     Why   are  overhead  electric  light  wires  seldom  smaller  than. 
No.  10  or  larger  than  No.  0000  B.  &  S.  guage? 

2.  What  is  "weather  proof  "  wire? 

3.  Why  is  No.  6  B.  &  S.  guage  wire  ordinarily  used  for  overhead  arc  light  circuits? 

4.  Why  is  the  ground  never  used  as  a  part  of  electric  lighting  circuits? 

5.  What  is  the  difference  between  a  "drawing  in"  and  a  "built  in"  system  of  under- 
ground conductors? 

6.  What  materials  are  used  for  insulating  electric  light  cables  which  are   intended 
for  use  in  underground  conduits? 

7.  What  is  "Edison  tubing"? 

8.  What  are  "feeders"?     What  are  "mains"? 

9.  Why  are  the  conductors  of  a  constant  pressure  electric  light  system  divided  into 
feeders  and  mains? 

10.  What  is  meant  by  inside  wiring? 

11.  What  is  meant  by  cleat  work?  by  moulding  work?  by  concealed  work? 

12.  Does  properly  arranged  electric  light  wiring  introduce  danger  from  fire  into  a 
building? 

13.  Does  improperly  arranged  electric  light  wiring  introduce  danger  from  fire  into  a 
building? 

14.  What  class  of  insulation  should  always  be  used  on  wires  for  inside  wiring? 

15.  Why  does  moulding  work,  when  properly  put  in,  make  the  best  kind  of  wiring? 

16.  Why  does  concealed  work,  where  the  wires  are  laid  directly  under  plaster,  make 
the  poorest  kind  of  wiring? 

17.  Why  is  it  advantageous  to  wire  buildings  on  the  distribution  plan? 

18.  Why  do  the  Underwriters'  rules  usually  prohibit  the  use  of  wires  smaller  in  size 
than  No.  16  B.  &  S.  gauge? 


XXIV. 

TESTING  ELECTRIC  LIGHT  CIRCUITS,  AND  THE  DIS- 
TRIBUTION AND  MEASUREMENT  OF  LIGHT. 

The  faults  which  occur  on  electric  light  lines  are  of  the  same 
kind  as  those  which  occur  on  telegraph  and  telephone  lines  (Lesson 
XVIII).  The  methods  of  testing  for  and  locating  the  faults  are  very 
different,  however.  The  general  condition  of  an  electric  light  line 
may  be  determined  from  the  manner  in  which  the  lights  burn. 
Breaks  in  the  line  are  made  evident  by  the  fact  that  lamps  on  the 
circuit  beyond  the  break  will  not  burn;  while  crosses  and  short  cir- 
cuits soon  make  themselves  evident  by  causing  the  fuses  which  pro- 
tect the  defective  part  of  the  circuit  to  melt  or  blow.  Poor  connec- 
tions may  be  shown  by  dimness  of  the  lamps,  when  the  connections 
have  a  sufficiently  high  resistance  to  cause  a  great  drop  in  pressure. 
It  is  needless  to  say  that  connections  or  joints  of  such  poor  conduct- 

205 


ivity  are  very  dangerous  and  should  not  be  permitted  to  exist  in  a 
circuit  for  an  hour.  All  joints  in  electric  light  wires  are  soldered  in 
order  that  there  may  be  no  "bad  joints"  which  may  cause  poor  con- 
nections. Poor  connection  at  such  points  as  sockets  or  fuse  blocks 
belonging  to  incandescent  circuits  may  cause  considerable  heating. 
If  such  heating  is  noticed  it  should  be  at  once  corrected  or  it  may 
cause  damage.  Sometimes  poor  connections  at  fuse  blocks  may 
cause  the  fuses  to  blow  when  there  is  really  no  trouble  elsewhere  on 
the  circuit.  This  may  occur  when  the  fuse  blocks  have  too  little 
contact  surface  at  the  connection  points  to  properly  carry  the  current. 
Such  fuse  blocks  should  always  be  replaced  by  larger  and  better  ones, 
as  they  are  not  only  an  annoyance  but  they  are  dangerous.  No  one 
would  think  for  a  moment  of  allowing  poorly  jointed  and  leaky  gas 
pipes  and  fixtures  in  a  house,  and  defective  electric  wires  should  be 
treated  in  exactly  the  same  manner  as  defective  gas  fittings. 

Series  circuits,  like  arc  light  circuits,  which  are  not  in  use  all 
through  the  twenty-four  hours,  are  often  tested  for  breaks,  grounds, 
and  crosses  by  means  of  a  magneto  bell  which  is  very  much  like  a 
telephone  call  bell  (Fig.  239).  The  little  magneto  machine  and  call 
bell  are  put  in  a  box  together  and  connected  in  series  with  two  ter- 
minals on  the  outside  of  the  box,  which  are  shown  at  the  top  of  the 
figure.  If  it  is  desired  to  test  the  continuity  of  a  line — that  is,  the 
absence  of  breaks — the  two  ends  of  the  line  are  connected  to  the  test 
bell  terminals.  If  the  bell  rings  when  the  crank  is  turned  the  cir- 
cuit is  shown  to  be  all  right,  while  if  the  bell  does  not  ring  the  cir- 
cuit is  shown  to  be  broken,  provided  the  test  bell  itself  is  in  good 
condition.  It  is  easy  to  test  the  latter  by  short  circuiting  the  ter- 
minals, when  the  bell  will  ring  upon  turning  the  crank  if  the  mag- 
neto is  all  right. 

If  it  is  desired  to  test  for  grounds  by  means  of  a  magneto  bell, 
one  terminal  of  the  bell  is  connected  to  earth  by  connecting  it  to  a 
gas  or  water  pipe,  jand  the  other  wire  is  connected  to  the  line  to  be 
tested.  If  the  bell  rings  when  the  crank  is  turned  it  ordinarily 
means  that  the  line  is  grounded,  and  if  the  bell  does  not  ring,  the 
line  is  shown  to  be  clear  of  grounds.  Sometimes  the  bell  will  ring 
a  little  when  the  line  has  a  very  high  insulation,  because  the  electro- 
static capacity  of  the  line  is  high  and  the  current  which  flows  into 
and  out  of  the  line,  as  it  is  charged  and  discharged  by  the  alternating 
pressure  set  up  by  the  magneto,  is  sufficient  to  ring  the  bell. 

Most  arc  light  lines  are  out  of  use  during  daylight, — only  those 
which  convey  current  to  arc  lamps  in  the  buildings  of  large  cities  are 
used  during  the  day, — and  many  lines  are  not  used  after  midnight.  It 
is  quite  convenient,  therefore,  to  use  the  magneto  bell  for  testing  such 
lines.  The  tests  can  be  made  each  day  an  hour  or  two  before  the 
lines  conie  into  service,  and  if  anything  is  found  to  be  wrong,  a  line- 
man can  go  along  the  line  to  find  the  trouble  and  fix  it. 


206 


FiG.  239.  FIG.  240. 

A  voltmeter  is  sometimes  used  for  testing  and  locating  grounds  on 
arc  light  lines  while  they  are  in  use.  Suppose  figure  240  to  represent 
an  arc  light  line  which  supplies  current  to  five  lamps  and  is  grounded 
at  F.  If  the  lamps  are  so  adjusted  that  each  requires  45  volts  press- 
ure, the  difference  of  pressure  between  the  fault  and  one  terminal  of 
the  dynamo  is  135  volts,  and  between  the  fault  and  the  other 
terminal  of  the  dynamo  the  difference  of  pressure  is  90  volts.  A  volt- 
meter connected  to  ground,  as  shown,  indicates  the  difference 
in  pressure  between  the  fault  and  one  dynamo  terminal,  and  so  shows 
between  which  lamps  the  fault  is  located.  Instead  of  using  a  volt- 
meter, 45  volt  incandescent  lamps  may  be  used  for  testing  by  this 
method.  As  many  45  volt  incandescent  lamps  are  connected  in 
series  as  there  are  arc  lamps  on  the  circuit  to  be  tested.  One  end  of 
the  series  is  connected  to  ground  and  the  other  to  one  dynamo 
terminal.  Then  one  incandescent  lamp  after  another  is  short  cir- 
cuited until  the  lamps  which  remain  in  the  circuit  burn  to  their  full 
candle  power.  The  number  of  incandescent  lamps  then  in  circuit  is 
equal  to  the  number  of  arc  lamps  between  the  dynamo  terminal  and 
the  fault.  The  reason  for  this  is  evident  upon  examining  the 
figure.  Since  there  are  three  arc  lamps  between  the  A  terminal  of 
the  dynamo  and  the  fault,  there  is  a  difference  of  pressure  of  135  volts 
between  the  two  points,  as  shown  by  the  voltmeter,  V.  135  volts  is 
the  pressure  required  to  bring  a  series  of  three  45  volt  incandescent 
lamps  to  full  candle  power,  so  that  the  number  of  arc  lamps  between 
the  fault  and  the  A  dynamo  terminal  is  equal  to  the  number  of  45 
volt  incandescent  lamps  which  will  burn  with  full  candle  power 
when  connected  in  series  between  the  dynamo  terminal  and  the 
ground.  This  test  is  made  upon  the  supposition  that  the  fault  has 
little  resistance  of  itself. 

In  testing  incandescent  circuits  for  grounds,  incandescent  lamps 
or  voltmeters  are  almost  always  used.  If  one  wire  of  a  two-wire  cir- 
cuit is  grounded  the  presence  of  the  ground  may  be  shown  by  con- 


207 


FIG.  241. 


FIG.  242. 


FIG.  243.  f 

necting  an  incandescent  lamp  between  the  other  wire  and  the  earth 
(water  pipes,  etc. ,  Fig.  241),  when  the  lamp  will  burn  on  account  of  the 
current  which  flows  from  one  wire  to  the  other  through  the  lamp  and 
the  fault.  If  the  lamp  be  intended  for  the  same  pressure  as  that  of  the 
circuit,  it  will  burn  at  full  candle-power  if  the  circuit  is  "dead 
grounded"  and  will  be  dimmer  in  proportion  to  the  resistance  of  the 
fault.  Figure  242  shows  a  permanent  arrangement  of  the  ground 
detector  which  is  fixed  so  that  the  detector  lamp  may  be  connected 
at  pleasure  with  either  of  the  wires.  Another  arrangement  of  lamps 
for  a  ground  detector  is  shown  in  figure  243.  A  and  B  are  two 
lamps  connected  in  series  between  the  two  wires  of  the  electric  light- 
ing system.  A  wire  goes  to  ground  through  a  fuse  block  and  a 
switch  from  a  point  between  the  two  lamps.  When  the  switch  is 
open  the  lamps  A  and  B  will  burn  very  dimly  but  of  equal  bright- 
ness, and  no  change  will  occur  when  the  switch  is  closed  if  no 
grounds  are  present  on  the  circuit.  If  the  wire  to  which  the  A  lamp 
is  connected  be  grounded,  current  will  flow  from  the  grounded  wire 


208 


FIG.  244. 


FIG.  245. 

through  the  B  lamp  to  the  other  wire  when  the  switch  is  closed,  and 
the  B  lamp  will  become  brighter  than  the  A  lamp.  In  the  same  way 
the  A  l^mp  will  brighten  when  the  switch  is  closed  if  the  B  wire  is 
grounded.  Sometimes  both  wires  are  grounded  and  the  faults  have 
about  equal  resistance.  In  this  case  the  lamps  will  not  show  the 
grounds  in  the  ordinary  way,  but  the  test  can  be  made  by  turning  off 
one  lamp  when  the  switch  is  closed  and  the  other  lamp  will  go  out 
if  the  circuit  is  not  grounded. 

For  three- wire  circuits  a  pair  of  lamps  may  be  used  as  a  ground 
detector  for  each  side  of  the  system. 

When  a  voltmeter  is  used  to  test  for  grounds  on  an  incandescent 
circuit  it  is  handled  in  very  much  the  same  way  as  the  incandescent 
lamD  which  is  used  for  the  same  purpose.  The  voltmeter  is  con- 
nected between  one  of  the  circuit  wires  and  the  earth.  If  the  other 
wire  of  the  circuit  is  grounded,  current  will  flow  from  it  through  the 
voltmeter  to  the  wire  to  which  the  instrument  is  connected.  If  the 
grounded  wire  is  "dead  to  ground,"  the  voltmeter  will  give  the  same 
reading  as  when  it  is  connected  directly  between  the  wires.  The 
reading  of  the  voltmeter  is  less  as  the  resistance  of  the  ground  con- 
tact is  greater,  and  it  is  zero  when  the  insulation  is  good. 

The  methods  which  are  used  for  testing  for  grounds  on  incan- 
descent circuits  show  when  a  ground  is  present  and  upon  which  wire 
it  exists,  but  they  do  not  give  any  clue  to  the  particular  portion  of 
the  circuit  upon  which  the  ground  is  located.  The  ordinary  method 
of  *  locating"  a  ground  which  cannot  be  found  by  inspection,  is  to 


209 


cut  one  branch  after  another  off  from  the  system  until  the  ground 
disappears.  The  ground  is  then  on  the  last  branch  cut  off  and  may 
be  found  by  careful  inspection. 

The  testing  of,  and  locating  faults  in,  lead  covered  cables  which 
aie  sometimes  used  in  underground  systems  is  done  in  the  same  way 
as  the  insulation  testing  of  telegraph  and  telephone  cables,  which  has 
already  been  explained  (Lesson  X,  page  72;  Lesson  XVIII,  page  145). 

The  candle-power  and  the  best  arrangement  of  the  lamps 
which  are  required  to  give  a*satisfactory  illumination  in  any  particu- 
lar space  can  be  determined  only  by  experience.  The  candle-power  of 
lamps  is  measured  by  an  instrument  called  a  photometer,  in  which 
the  illuminating  power  of  the  lamp  to  be  measured  is  directly  com- 
pared with  the  power  of  standard  candles  (Lesson  XXI,  page  175), 
or  with  a  gas  jet  or  lamp  of  known  candle-power.  The  commonest 
form  of  a  photometer  is  that  called  Bunsen's  photometer,  which  is 
shown  diagrammatically  in  Fig.  244.  A  is  the  standard  candle,  B  is 
the  lamp  whose  illumination  is  to  be  measured,  and  D  is  a  movable 
disc  of  thin  paper  with  a  grease  spot  at  its  center.  The  photometer, 
for  practical  use,  is  all  enclosed  in  a  perfectly  dark  closet,  and  the 
light  from  A  and  B  is  carefully  screened  on  every  side  except  directly 
in  line  with  the  disc.  An  observer  measures  the  unknown 
candle-power  of  the  lamp  B  by  moving  the  disc  D  until  it 
shows  an  equal  illumination  on  both  sides.  The  disc  is  generally 
looked  at  by  means  of  mirrors,  so  that  both  sides  may  be  seen  at 
once.  When  the  illumination  of  the  two  sides  of  the  disc  is  equal, 
the  candle-powers  of  the  lights  are  proportional  to  each  other  in  the 
ratio  of  the  squares  of  the  distances  measured  from  the  respective  lights 
to  the  disc.  , 

The  reason  that  the  squares  of  the  distances  come  into  the  com- 
parisons of  candle-powers  is  illustrated  in  Fig.  245.  If  we  suppose  a 
screen,  A  B,  to  be  placed  at  a  distance  of  one  foot  from  the  lamp,  L, 
we  may  consider  that  the  screen  is  illuminated  by  a  certain  number 
of  rays  of  light  falling  upon  it.  Now,  if  the  screen  be  moved  to  a 
distance  of  two  feet  from  the  lamp  the  same  rays  of  light  will  illumin- 
ate an  area,  C  D,  which  is  four  times  as  large  as  A  B,  and,  conse- 
quently, the  intensity  of  the  illumination  on  the  screen  is  only 
one-fourth  as  great  as  when  the  screen  was  at  a  distance  of  one  foot 
from  the  lamp.  If  the  screen  be  moved  to  a  point  three  feet  from 
the  lamp,  the  same  rays  will  cover  the  area  E  F,  which  is  nine  times 
as  large  as  A  B,  and  the  intensity  of  the  illumination  is  only  one- 
ninth  as  great  as  when  the  screen  was  within  a  foot  of  the  lamp. 
Since  4  and  9  are  respectively  equal  to  the  squares  of  2  and  3,  we  see 
that  the  intensity  of  the  illumination  given  to  a  surface  by  a  fixed 
light  is  inversely  proportional  to  the  square  of  the  distance  from  the  light 
to  the  surface.  In  the  Bunsen  photometer,  the  screen  is  placed  at  such  a 


point  directly  between  two  lights  that  they  illuminate  it  equally.  In 
this  case,  the  lights  must  have  candle-powers  which  are  proportional 
to  the  squares  of  their  distances  from  the  screen,  as  already  said. 

The  actual  illuminating  effect  of  a  given  number  of  lamps  in 
any  space  depends  upon  a  great  many  things.  For  instance,  a  room 
with  dark  walls,  which  absorb  a  great  deal  of  light,  requires  much 
more  light  to  give  a  satisfactory  illumination  than  does  a  room  with 
light-colored  or  white  walls.  In  a  comparatively  small  space  a  num- 
ber of  lamps  of  small  candle-power,  properly  distributed  about  the 
space,  give  a  much  more  satisfactory  light  than  do  a  few  large  lamps 
giving  the  same  total  candle-power.  This  is  because  the  illumina- 
tion near  the  large  lamps  is  very  great  and  at  other  points  in  the 
space  the  illumination  is  small,  while  it  is  much  more  uniformly 
distributed  by  the  small  lamps. 

In  ordinary  rooms  and  stores,  it  is  common  to  put  from  one  to 
three  16  candle-power  lights  for  each  100  square  feet  of  floor,  while 
in  larger  rooms  450  watt  arc  lamps  may  be  used  so  that  each  arc 
illuminates  from  500  to  1,000  square  feet  of  floor.  Where  arcs 
are  placed  indoors,  it  is  usual  to  surround  the  arc  with  an  opal  glass 
globe  which  distributes  the  light  more  satisfactorily  than  would 
otherwise  be  the  case.  Such  globes  have  the  disadvantage  of  ab- 
sorbing nearly  one-half  of  the  light  of  the  arc,  but  their  effect  in  dis- 
tributing the  light  is  sufficiently  important  in  indoor  lighting  to 
counterbalance  the  loss  of  light.  For  outdoor  lighting,  arc  lights 
with  clear  glass  globes  are  used  almost  altogether.  The  lamps  are 
then  placed  from  50  to  600  feet  apart,  depending  upon  the  amount  of 
illumination  desired.  It  is  an  important  fact  which  is  not  very  well 
known  by  electric  light  companies,  that  dirty  globes  of  clear  glass 
may  absorb  even  more  light  than  do  opal  globes,  so  that  it  is  impor- 
tant that  arc  light  globes  be  kept  clean. 

The  true  measure  of  illumination,  it  may  be  seen  from  what 
precedes,  is  not  the  candle-power  but  the  amount  of  light  or  illumin- 
ation obtained  on  a  surface,  and  the  unit  for  measuring  illumination 
is  the  amount  of  illumination  on  a  perpendicular  screen  at  the  dis- 
tance of  one  foot  from  a  lamp  giving  one  candle-power.  Four  candle- 
power  at  a  distance  of  two  feet  from  the  screen  and  9  candle-power 
at  a  distance  of  three  feet  from  the  screen  give  the  same  illumination 
as  i  candle-power  at  a  distance  of  one  foot  from  the  scren,  which  is 
called  a  candle-foot.  The  illumination  given  by  any  lamp  upon 
a  perpendicular  surface  is  equal  to  the  candle-power  of  the  lamp 
divided  by  the  square  of  the  distance  between  the  lamp  and  the  sur- 
face. For  instance,  if  we  have  a  32  candle-power  lamp  at  a  distance 
of  6  feet  from  a  wall,  the  illumination  on  the  wall  is  32  divided  by  6 
squared  or  36,  which  is  equal  to  about  .9  of  a  candle-foot.  An  illum- 
ination of  i  candle-foot  is  quite  satisfactory  for  ordinary  reading. 
Ordinary  bright  moonlight  gives  an  illumination  on  the  ground 


which  is  equal  to  about  rf  & t  of  a  candle-foot.  The  illumination  upon 
theatre  stages  is  ordinarily  from  3  to  4  candle-feet,  and  the  illumin- 
ation given  by  diffused  daylight  is  equal  to  from  10  to  40  candle- 
feet.  On  account  of  the  expense  of  producing  artificial  light  by  the 
common  methods  of  the  present  day,  it  is  commercially  impracticable 
to  artificially  produce  as  great  an  illumination  as  is  given  by  day- 
light. 

Coprighted,  1895, 


The  National  School  of  Electricity. 

REVIEW  OF  LESSON  XXIV. 

Points  for  Review.     1.     What  faults  occur  on  electric  light  circuits? 

2.  Why  is  it  necessary  to  solder  the  joints  in  electric  light  wires? 

3.  How  is  a  magneto  bell  used  in  testing  electric  circuits? 

4.  How  may  a  voltmeter  be  used  to  "  locate"  grounds  on  arc  light  circuits? 

5.  How  may  incandescent  lamps  be  used  for  the  same  purpose? 

6.  What  is  a  "  ground  detector?" 

V.     How  are  incandescent  lamps  used  for  ground  detectors  on  incandescent  circuits? 

8.  How  may  a  voltmeter  be  used  to  test  for  grounds  on  incandescent  circuits? 

9.  How  are  grounds  ordinarily  "  located"  on  incandescent  circuits? 

10.  How  are  faults  located  on  underground  cables? 

11.  What  is  a  photometer?     What  are  standard  candles? 

12.  Upon  what  does  the  illuminating  effect  of  the  lamps  in  a  room  depend? 

13.  W'hat  effect  does  an  opal  globe  have  on  the  light  given  from  a  lamp?  What  effect 
does  a  dirty  globe  of  clear  glass  have? 


XXV. 

ELECTROMAGNETIC  INDUCTION. 

It  has  been  experimentally  proved  that  any  change  in  the  mag- 
nectic  field  around  an  electric  conductor  which  causes  the  lines  of 
force  to  cut  the  conductor  tends  to  cause  an  electric  current  to  flow 
in  the  conductor  (Lesson  XIX,  page  147).  We  are  now  sufficiently 
acquainted  with  the  mutual  effects  of  electric  currents  and  magnetism 
(Lessons  V,  VI,  XIX  and  XX)  so  that  it  is  not  surprising  that 
there  are  various  conditions  under  which  the  effects  of  magnetism  may 
result  in  an  electric  current.  One  of  these  conditions  is  seen  in 
dynamos  where  the  motion  of  the  armature  cbnductors  across  mag- 
netic lines  of  force  causes  a  current  to  flow  in  the  electric  circuit 
of  which  the  armature  is  a  part  (Lessons  XIX  and  XX).  It  is  not 
necessary  that  the  conductors  move,  but  the  magnetic  field  may  move 
so  that  its  lines  offeree  cut  across  stationary  conductors.  In  fact,  an 
electric  pressure  is  set  up  in  a  conductor  when  it  cuts  lines  of  force, 
whether  the  cutting  be  caused  by  the  motion  of  the  conductor  or  by  the 
motion  of  the  lines  of  force. 

213 


The  magnetic  lines  of  force  which  are  cut  by  a  conductor  and 
so  cause  an  electric  pressure  in  the  conductor  may  not  come  from  a 
magnet,  but  may  belong  to  an  electric  current  in  a  neighboring  wire. 
When  a  conductor  is  moved  towards  or  away  from  a  wire  carrying  a 
current,  the  lines  of  force  belonging  to  the  current  are  cut  by  the 
moving  conductor  and  an  electric  pressure  is  induced  in  it.  If  the 
wire  carrying  the  current  be  moved  towards  or  away  from  the  other 
conductor,  the  lines  of  force  belonging  to  the  current  are  cut  by  the 
conductor  which  is  now  stationary,  and  an  electric  pressure  is  set  up 
as  before.  The  wire  carrying  the  current  may  be  in  the  form  of  a  coil, 
like  P  in  Fig.  246.  An  electric  pressure  may  be  set  up  in  the  conduc- 
tors of  another  coil,  S,  by  simply  thrusting  the  first  coil  which  carries 
a  current  into  the  second.  '  After  the  primary  coil,  P,  is  pushed 
into  the  secondary  coil,  S,  and  its  movement  is  stopped,  the  electric 
current  in  the  secondary  also  stops  because  the  conductors  no  longer 
cut  lines  of  force  and  the  electrical  pressure  is  no  longer  produced. 
Now  if  the  primary  coil  be  drawn  out  from  the  secondary  coil,  an 
electrical  pressiire  is  again  set  up  in  the  latter.  This  pressure  and 
its  resulting  current  is  opposite  to  that  set  up  when  the  primary  coil  was 
pushed  into  the  secondary,  because  the  lines  of  force  are  cut  in  the  oppo- 
site direction  by  the  secondary  coil.  The  same  effects  may  be  pro- 
duced by  moving  a  secondary  coil  in  and  out  of  a  larger  primary  coil 
or  by  moving  one  coil  around  near  the  other.  The  battery,  C,  shown 
in  Fig.  246,  furnishes  current  to  the  primary  coil. 

The  same  effects  may  be  produced  by  permanently  fixing  the 
coil  P  inside  of  the  coil  S,  and  then  varying  the  current  which  flows 
through  the  coil  P.  When  the  current  increases  in  the  primary^coil, 
the  lines  of  force  belonging  to  the  magnetic  field  of  the  current  cut 
the  conductors  of  the  secondary  coil  as  they  are  produced,  and  thus 
set  up  an  electric  pressure  in  the  secondary  coil  during  the  time 
the  magnetic  field  is  increasing.  If  the  primay  current  be  reduced  or 
shut  off  entirely,  an  electric  pressure  is  set  up  in  the  secondary  coils 
in  the  opposite  direction  during  the  time  that  the  magnetic  field  is 
decreasing. 

Electric  currents  which  are  set  up  in  circuits  by  means  of  cutting 
lines  of  force  are  said  to  be  caused  by  electromagnetic  induction,  and 
the  currents  are  sometimes  spoken  of  as  induction  currents  or  induced 
currents.  The  currents  produced  by  dynamos  are  examples  of  cur- 
rents induced  by  electromagnetic  action. 

An  appliance  consisting  of  a  primary  coil  and  a  secondary 
coil,  which  is  used  for  the  purpose  of  inducing  currents  in 
the  circuit  of  the  secondary  coil  by  varying  the  current  in  the 
primary  coil,  is  called  an  induction  coil  (see  Lesson  XVI,  page 
123).  The  two  windings  of  an  induction  coil  are  usually  placed  on  an 
iron  core  which  greatly  increases  their  effectiveness.  The  core 


214 


J 


FIG.  246. 


FIG.  247. 


FIG.  248. 


215 


must  be  made  of  iron  wires,  or  eddy  currents  will  be  induced 
in  the  core  and  thus  heating  and  loss  of  power  will  result,  since 
currents  are  induced  in  all  closed  circuits  or  masses  of  metal 
which  are  in  a  changing  magnetic  field.  '  The  division  of  the  iron 
core  of  an  induction  coil  is  thus  seen  to  be  necessary  for  the  same 
reason  that  it  is  necessary  to  laminate  the  iron  cores  of  dynamo 
armatures  (L,esson  XX,  page  156). 

Each  turn  of  the  secondary  windings  of  a  well  built  induction 
coil  cuts  practically  all  of  the  lines  of  force  which  are  set  up  by  the 
current  in  the  primary  coil,  so  that  the  total  electrical  pressure 
induced  in  the  secondary  windings  may  be  controlled  by  winding  the 
secondary  coil  with  a  greater  or  less  number  of  turns  of  wire.  The 
induction  coil  used  with  a  telephone  transmitter  is  arranged  to  give 
a  fairly  high  pressure  in  the  secondary  coils  and  thus  intensify  the 
effect  of  a  single  cell  of  battery.  In  the  induction  coils  made  for 
scientific  experiments,  which  are  often  called  RuhmkorfF  coils  (Fig. 
247),  the  secondary  has  so  very  many  turns  of  extremely  fine  wire 
that  the  pressure  produced  in  the  secondary,  when  the  current  from 
a  few  battery  cells  is  made  and  broken  in  the  primary  coil,  may  be 
so  great  as  to  cause  an  electric  spark  to  jump  a  number  of  inches 
through  air.  In  the  induction  coils  commonly  called  transformers 
or  converters  (Fig.  248),  which  are  common  objects  on  the  poles  of 
electric  light  companies  which  use  alternating  currents,  the  secondary 
coils  usually  have  fewer  turns  than  have  the  primary  coils,  and  the 
electrical  pressure  induced  in  the  secondary  coils  is  therefore  less  than 
the  pressure  applied  to  the  primary.  Transformers  are  used  to  reduce 
a  high  pressure  which  is  used  on  the  distributing  circuits  to  a  lower 
pressure  which  may  be  safely  and  conveniently  used  in  buildings  to 
operate  electric  lights.  Transformers,  as  applied  to  electric  lighting, 
will  receive  attention  in  later  lessons.  By  means  of  them  we  are  able 
to  perform  the  remarkable  feat  of  commercially  transferring  electrical 
power  from  one  circuit  to  another,  although  the  circuits  have  abso- 
lutely no  electrical  connection  with  each  other. 

If  we  remember  the  direction  of  the  lines  of  force  around  a  wire 
which  carries  a  current  (Lesson  VI,  page  36),  and  the  rule  for  de- 
termining the  direction  of  an  induced  current  (Lesson  XIX,  page  151), 
it  is  easy  to  determine  the  direction  of  the  current  induced  in  any 
secondary  circuit.  By  applying  the  rules  referred  to,  the  following 
rules  relating  to  induced  currents  may  be  arrived  at: 

1 .  When  a  primary  coil  is  PUSHED  INTO  a  secondary  coil,  the 
secondary  induced  current  is   OPPOSITE  IN  DIRECTION  to  the 
primary    current. 

2.  When  a  primary  coil  is  DRAWN  OUT  of  a  secondary  coil, 
the  induced  secondary  current  is  in  the  SAME  DIRECTION  as  the 
primary  current. 


When  the  primary  and  secondary  coil  are  fastened  together  and 
current  is  induced  in  the  secondary  by  making  and  breaking  the 
primary  current  we  have  the  following  rules: 

3.  When  the  current  is  MADE  (started}  in  the  primary  coil, 
a  momentary  OPPOSITE  or  INVERSE  current  is  induced  in  the 
secondary  coil. 

4.  When  a  current  is  BROKEN  (stopped)  in  the  primary  coil, 
a  momentary  current  of  the  SAME  DIRECTION  is  induced  in  the 
secondary  coil. 

These  rules  relate  to  the  flow  of  current  when  the  secondary 
circuit  is  closed.  If  the  secondary  circuit  be  open,  the  electrical 
pressure  which  is  set  up,  is  in  such  a  direction  that  the  current  would 
flow  in  the  direction  indicated  were  the  circuit  closed. 

A  careful  examination  of  these  rules  shows  a  very  important  fact 
which  may  be  stated  in  this  way:  The  direction  of  an  induced  cur- 
rent is  always  such  that  the  magnetic  field  set  up  by  it  tends  to  oppose 
the  change  in  the  strength  of  the  magnetic  field  belonging  to  the  pri- 
mary current.  For  instance,  when  the  primary  current  of  an  induc- 
tion coil  is  "made,"  an  inverse  current  is  induced  in  the  secondary 
coil  whose  magnetic  field  opposes  the  growth  -of  the  magnetic 
field  of  the  primary  current.  When  the  primary  circuit  is 
broken,  the  magnetic  field  of  the  induced  current  opposes  the  decay 
of  the  magnetic  field  belonging  to  the  primary  current.  Another 
illustration  may  be  taken  from  the  primary  coil  which  is  pushed  into 
a  secondary  coil.  When  the  primary  coil  carrying  a  current  is  pushed 
into  the  secondary,  an  inverse  current  is  induced  which  sets  up  a  mag- 
netic field  which  tends  to  repel  the  primary  coil  and  therefore  opposes 
its  motion.  When  the  primary  coil  is  drawn  out  of  the  secondary  the 
direct  induced  current  sets  up  a  magnetic  field  which  tends  to  attract 
the  primary  coil  and  therefore  again  opposes  its  motion. 

In  the  case  of  a  dynamo  the  current  which  is  induced  in  the 
armature  conductors  has  such  a  direction  that  its  magnetic  effect 
tends  to  stop  the  motion  of  the  armature  (Lesson  XIX,  page  147); 
and  to  keep  it  rotating,  mechanical  power  must  be  applied  to  the 
armature  in  proportion  to  the  amount  of  power  represented  by  the 
currents  taken  from  the  armature  (Lesson  VIII,  page  52). 

The  above  facts  may  be  briefly  stated  in  one  sentence.  When 
electric  currents  are  induced  by  a  changing  magnetic  field,  the  mag- 
netic field  belonging  to  the  induced  currents  tends  to  stop  the  change  in 
ike  original  field.  We  have  also  the  following  statement  which  re- 
sults directly  from  the  former:  When  electric  currents  are  induced  by 
the  motion  of  a  conductor,  the  induced  currents  have  such  a  direction  that 
their  magnetic  effect  tends  to  stop  the  motion.  This  is  called  Lenz's 
law,  after  a  German  scientist  who  first  formally  stated  the  principle. 


The  principles  stated  in  the  preceding  paragraph  are  a  direct  re- 
sult of  the  general  law  of  the  Conservation  of  Energy  (Lesson  VIII, 
page  52).  We  can  transform  mechanical  energy  into  electrical  energy, 
or  vice  versa,  or,  we  can  transform  the  energy  of  electrical  currents 
flowing  under  one  pressure  into  the  energy  of  electrical  currents  flow- 
ing under  another  pressure,  but  in  every  case  as  much  energy  must 
be  put  into  the  transforming  apparatus — whether  it  be  dynamo,  mo- 
tor, Ruhmkorff  coil  or  transformer — as  is  taken  out.  We  have  already 
seen  that  the  useful  "output"  of  electrical  apparatus  is  usually  smaller 
than  the  ( 'input' '  by  a  certain  percentage  of  the  total  energy  which 
has  been  changed  into  useless  heat  (Lesson  VIII,  page  52). 

A  varying  current  may  have  an  inductive  effect  upon  the  coil  in 
which  it  flows  itself,  in  addition  to  its  inductive  effect  upon  adjacent 
conductors.  When  a  current  is  started  in  a  coil  it  sets  up  a  magnetic 
field  which  quickly  grows  from  zero  to  its  full  value.  As  the  field 
grows,  its  lines  of  force  cut  the  turns  of  the  coil  and  induce  in  them 
an  electric  pressure  which  opposes  the  growth  of  the  current.  On 
stopping  the  original  current  its  magnetic  field  quickly  dies  away  and 
the  lines  of  force  again  cut  the  turns  of  the  coil,  but  this  time  in 
such  a  direction  that  the  self-induced  electric  pressure  upholds  the 
original  current.  If  the  coil  has  a  great  many  turns  wound  on  an 
iron  core,  its  self-induction  may  be  of  sufficient  magnitude  to  make 
a  brilliant  spark  or  give  a  severe  shock  when  the  circuit  is  broken. 
The  spark  at  breaking  a  circuit  is  often  spoken  of  as  caused  by  the 
extra  current  of  self-induction.  The  effect  of  self-induction  is  made 
use  of  in  so-called  spark  coils  which  are  used  with  devices  for  light- 
ing gas  by  electricity,  and  which  consist  simply  of  a  coil  containing 
many  turns  of  insulated  wire  wound  on  a  core  of  iron  wire.  The 
effect  of  self-induction  makes  itself  evident  if  the  circuit  of  a  single 
battery  cell  be  broken  between  the  hands  when  the  circuit  contains 
a  spark  coil,  telegraph  instrument,  or  other  electromagnetic  coil. 

The  fact  that  a  conductor  carrying  an  electric  current  is  always 
surrounded  by  a  magnetic  field  (Lesson  VI,  page  38),  would  lead  us 
to  expect  conductors  carrying  electric  currents  to  attract  and  repel 
each  other.  This  is  indeed  the  fact.  We  have  already  seen  that 
solenoids  act  towards  each  other  exactly  as  though  they  were  mag- 
nets (Lesson  VI,  page  38).  In  every  case,  we  have  learned  that  when 
magnets  or  solenoids  are  brought  into  each  other's  influence,  they  tend 
to  move  so  that  their  lines  of  force  shall  be  placed  parallel  and  in  the 
same  direction  (Lesson  VI,  page  38).  Exactly  the  same  is  true  of 
straight  or  curved  wires  which  are  brought  into  each  other's  influence. 
Remembering  this,  we  can  see  that  two  wires  lying  side  by  side  must  at- 
tract each  other  if  they  carry  currents  flowing  in  the  same  direction. 
This  is  because  the  lines  of  force  can  only  become  parallel  and  of  the 
same  direction  when  the  two  conductors  are  very  close  together. 


FIG.  249. 


FIG.  250. 

When  the  currents  flow  in  opposite  directions  the  wires  repel  each 
other.  In  the  same  way,  if  the  wires  are  inclined  to  each  other  they 
tend  to  turn  around  into  such  a  position  that  the  wires  are  parallel 
and  the  currents  flow  in  the  same  direction  (Fig.  249).  It  is  upon 
this  principle  that  the  electrodynamometer  acts  (Lesson  XI,  page 
78).  The  operation  of  Kelvin  balances  (Fig.  250)  which  have  been 
explained  in  Lesson  XI,  is  based  upon  the  attraction  and  repulsion 
of  parallel  currents.  These  instruments,  as  stated  in  Lesson  XIV, 
are  not  sufficiently  portable  to  bring  them  into  common  use,  but  they 
are  excellent  for  use  as  standards  by  which  to  calibrate  commercial 
instruments. 

Copyrighted,  1895, 

210 


The  National  School  of  Electricity. 

REVIEW    OF  LESSON  XXV. 

Points  for  Review.     1.     What  is  the  effect  of  cutting  magnetic  lines  of  force  by  an 
electric  conductor? 

2.  What  is  the  difference  in  the  result  when   the  conductor  is  part  of  a  closed  elec- 
tric circuit,  and  when  it  is  a  part  of  an  open  circuit? 

3.  Is  it  necessary  for  the  conductor  to  move  in  order  that  it  may  cut  lines  of  force? 

4.  If  a  magnet  or  a  primary  coil  is  pushed  into  a  secondary  coil,  does  the  inductive 
effect  last  after  the  motion  has  ceased? 

5.  What  are  "induced currents"?     What  are  induction  coils? 

6.  Why  cannot  a  solid  iron  bar  be  used  for  the  core  of  an  induction  coil? 

7.  How  is  it  possible  to  alter  the  electrical  pressure  induced  in  the  secondary  wind- 
ings of  an  induction  coil? 

8.  What  are  transformers  or  converters? 

9.  What  is  the  general  rule  for  the  direction  of  an  induced  current? 

10.  Is  it  possible  to  take  a  greater  amount  of  power  out  of  the  secondary  of  an  induc- 
tion coil  or  transformer  than  is  put  into  the  primary? 

11.  What  is  self-induction? 

12.  Why  does  self-induction  cause  a  spark  upon  breaking  an  electric  circuit? 

13.  Why  do  parallel  wires  carrying  currents  which  flow  in  the  same  direction  attract 
each  other? 

14.  Why  do  parallel  wires  carrying  currents  which  flow  in  opposite  directions  repel 
each  other? 


XXVI. 

ALTERNATING  CURRENTS. 

We  have  already  learned  (Lesson  XIX,  page  151)  what  an  alter- 
nating current  is,  and  how  it  may  be  produced  in  an  armature  hav- 
ing a  single  coil  of  wire  which  is  revolved  between  two  pole  pieces. 
The  ordinary  alternating  current  dynamo  or  alternator  is  made  up  on 
this  principle,  but  is  usually  constructed  with  a  number  of  coils  on 
the  armature  and  with  an  equal  number  of  poles  in  the  field  magnets. 
In  general  construction  an  alternator  is  similar  to  a  continuous  cur- 
rent dynamo,  but  before  we  enter  into  a  discussion  of  the  detailed 
differences  it  is  well  to  consider  certain  facts  in  regard  to  the  alternat- 
ing current. 

If  a  pulsating  current  which  varies  in  value  like  that  represented 
in  Fig.  161  (see  Lesson  XIX,  page  153)  be  passed  through  a  volta- 
meter (Lesson  IX,  page  62),  the  amount  of  metal,  copper,  for  instance, 
which  is  carried  by  the  current  from  the  anode  to  the  cathode  is  pro- 
portional to  the  average  value  of  the  current.  In  other  words,  the 
electrolytic  effect  of  a  pulsating  current  is  dependent  upon  the  average 

220 


or  mean  value  of  the  current.  The  electrolytic  effect  of  the  pulsating 
current  represented  by  Fig.  161,  is  the  same  as  that  of  the  uniform 
current,  the  magnitude  of  which  is,  represented  in  Fig.  251  by  the 
height  of  the  line  FG  above  the  line  AB.  If  the  pulsating  current 
of  Fig.  161  had  not  been  commutated,  but  had  been  led  into  the 
external  circuit  by  means  of  collecting  rings  (Fig.  157,  L,esson  XIX) 
as  is  done  in  telephone  magnetos,  the  second  loop  of  the  curve  rep- 
resenting the  current  would  fall  below  the  line  AE,  because  the  cur- 
rent flows  alternately  in  one  direction  and  then  in  the  other.  This  is 
shown  in  Fig.  252,  where  the  perpendicular  distances  from  the  line  OX 
to  the  wavy  line  are  proportional  to  the  strength  of  the  current  in  the 
circuit  at  each  instant.  During  the  times  represented  by  the  distances 
AC  etc.,  in  which  the  loops  are  above  the  line  OX,  the  current  is 
supposed  to  flow  in  one  direction,  and  during  the  intervening  times, 
CE,  etc.,  in  which  the  loops  are  below  the  line  OX,  the  current  is 
supposed  to  flow  in  the  other  direction.  Such  an  alternating  current 
can  have  no  electrolytic  effect,  except  under  exceptional  circumstances, 
since  the  electric  current  which  flows  in  one  direction  for  one  instant 
flows  in  the  opposite  direction  for  the  next  instant  and  consequently 
the  voltameter  plates  are  alternately  anode  and  cathode. 

A  different  condition  exists  in  regard  to  the  heating  effect  of  a 
pulsating  or  alternating  current.  It  is  to  be  remembered  that  the 
heating  produced  by  a  continuous  current  when  it  flows  through  a 
circuit,  is  equal  to  the  current  squared  multiplied  by  the  resistance  of 
the  circuit  (Lesson  VIII,  page  53).  The  heating  produced  by  a  pulsat- 
ing current  is  equal  at  every  instant  to  the  value  of  the  currrent  at 
that  instant  squared  and  multiplied  by  the  resistance  of  the  circuit. 
A  curve  may  be  drawn,  as  is  shown  in  Fig.  253,  the  height  of 
which  at  each  point  is  equal  to  the  square  of  the  corresponding 
height  of  the  curve  representing  the  current.  The  height  of  this 
curve  of  squares  at  each  point  is  proportional  to  the  power  expended 
in  heating  the  circuit  at  the  corresponding  instant.  The  same  total 
power  would  be  expended  in  the  circuit  by  a  continuous  current 
whose  square  is  equal  to  the  average  height  of  the  curve  of  squares. 
In  Fig.  254  the  line  AbCdE  represents  the  curve  of  squares  as 
already  shown  in  Fig.  253,  and  the  height  of  the  line  FG  above  OX 
represents  the  square  of  the  continuous  current  which  causes  the 
same  heating  in  the  circuit  as  the  pulsating  current.  The  height  of 
the  line  FG  is  greater  than  the  square  of  the  average  value  of  the 
pulsating  current,  and  consequently  the  heating  effeU  of  a  pulsating 
current  is  greater  than  that  of  a  continuous  current  equal  to  its 
average  value. 

The  reason  for  this  fact  may  be  easily  seen.  The  squares  of  num- 
bers increase  in  magnitude  much  more  rapidly  than  do  the  numbers 
themselves.  For  instance,  6  is  twice  3,  but  the  square  of  6,  or 


./•v  /*\  , 
7    V    \ 


C 

FIG,  251. 


FIG.  252. 


FIG.  254. 


FIG.  253. 


FIG.  255. 


FiG.  256. 


FIG.  257. 


36,  is  four  times  the  square  of  3,  or  9.  On  account  of  this,  the  aver- 
age of  the  squares  of  different  positive  numbers  is  always  greater 
than  the  square  of  the  average  of  the  numbers.  For  instance,  the 
average  of  2,  5  and  8  is  15  divided  by  3,  or  5,  and  its  square  is  25. 
The  squares  of  these  numbers  are  respectively  4,  25  and  64,  which 
give  an  average  of  93  divided  by  3,  or  31.  Now  if  we  square  the 
values  of  the  pulsating  current  at  each  instant  we  have  the  squares  of 
a  large  number  of  values  which  range  from  zero  to  a  maximum,  and 
the  average  of  these  squares  is  greater  than  the  square  of  the  average 
of  the  original  values. 

Since  the  heating  effect  of  a  current  is  entirely  independent  of  its 
direction,  an  alternating  current  such  as  that  represented  by  Fig.  251 
expends  exactly  the  same  power  in  heating  a  circuit  or  given  resistance 
as  it  would  if  commutated  into  a  pulsating  ctirrent. 

When  there  is  no  self-induction  or  outside  disturbing  factor  in  a 
circuit,  the  power  expended  in  the  circtiit  is  always  equal  to  CB 
(current  times  electric  pressure).  Here,  again,  when  the  pressure 
and  resulting  current  are  pulsating  or  alternating,  we  have  a  series  of 
products  of  values,  the  average  of  which  is  greater  than  the  product 
of  the  respective  averages  of  the  current  and  pressure.  The  line 
ABCDB,  in  Fig.  255,  represents  the  electric  pressure  applied  in  a 
circuit,  and  AbCdE  the  resulting  current.  At  each  instant  the 
power  expended  in  sending  the  current  through  the  circuit  is  equal  to 
the  product  of  the  corresponding  heights  of  these  two  curves.  The 
height  of  the  curve  APCQE  at  each  point  is  equal  to  the  product  of 
the  corresponding  heights  of  the  current  and  pressure  curves.  Curve 
APCQE  may  therefore  be  called  a  power  curve.  Both  its  loops 
are  placed  above  the  line  OX  because  they  both  represent  power  ex- 
pended in  the  circuit.  The  average  power  expended  in  the  circuit 
is  represented  by  the  height  of  the  line  FG,  which  cuts  off  the  tops 
of  the  loops  so  that  they  will  exactly  fill  up  the  intervening  valleys. 
The  height  of  the  line  HJ  represents  the  product  of  the  average 
current  by  the  average  pressure,  which  is  seen  to  be  less  than  the 
average  power  represented  by  the  height  of  the  line  FG. 

When  we  measure  the  value  of  an  alternating  current  we  desire 
to  find  the  value  which,  when  squared  and  multiplied  into  the  resist- 
ance of  a  circuit,  will  give  the  heating  effect  of  the  current.  This  is 
called  the  effective  value  of  the  current  or  the  effective  current,  and  it 
is  greater  than  the  average  value  of  the  current,  as  we  have  already  seen. 
In  measuring  an  alternating  electric  pressure  or  electromotive  force  we 
likewise  desire  to  find  the  value  which,  when  multiplied  into  the  effect- 
ive current  which  it  causes  to  flow  through  a  circuit  without  self- 
induction,  will  give  the  power  expended  in  the  circuit.  This  is 
called  the  effective  pressure  or  effective  electromotive  force,  and  is 
larger  than  the  average  pressure.  From  the  explanation  given  above 


223 


it  is  seen  that  the  effective  value  of  an  alternating  current  or  an  alter- 
nating electric  pressure  is  equal  to  the  square  root  of  the  average  of 
all  the  squares  of  the  instantaneous  values  of  the  current  or  pressure 
during  the  time  represented  by  one  loop  in  the  figures.  The  effective 
value  is,  therefore,  often  spoken  of  as  the  u  square  root  of  the  mean 
(average)  square. ' ' 

Since  the  indications  of  an  electro-dynamometer  or  of  a  hot  wire 
electrical  measuring  instrument  are  proportional  to  the  square  of  the 
current  flowing  through  the  instrument  (Lesson  XI,  page  78)  they  are 
excellently  adapted  to  measuring  alternating  currents.  The  number 
of  alternations  of  direction  made  in  each  minute  by  the  alternating 
currents  which  are  ordinarily  used  is  so  great  that  the  movable  coil 
of  an  electro-dynamometer  acts  exactly  as  though  it  were  pulled 
around  by  a  continuous  force  proportional  to  the  averageof  the  squares 
of  the  instantaneous  values  of  the  current.  The  square  root  of  the 
indications  of  the  instrument  is  therefore  proportional  to  the  effective 
value  of  the  alternating  current  flowing  through  its  coils.  One  form 
of  electro-dynamometer  which  is  commonly  used  for  measuring  alter- 
nating currents  is  shown  in  Fig.  56  (Lesson  XI).  Another  form  is 
shown  in  Fig.  256.  Alternating  current  voltmeters  made  upon  the 
same  principle  are  shown  in  Figs.  256  and  257. 

Since  hot  wire  instruments  also  average  up  the  squares  of  the 
instantaneous  values  of  the  current,  they  have  been  used  to  some  ex- 
tent as  instruments  for  measuring  alternating  currents.  The  Cardew 
voltmeter  shown  in  Fig.  60  (Lesson  XI),  which  is  made  upon  this  prin- 
ciple, has  been  much  used  as  an  alternating  current  instrument.  Elec- 
trostatic voltmeters  (Lesson  XI,  page  82)  also  give  indications,  the 
square  roots  of  which  are  proportional  to  effective  alternating  pressures 
when  the  needle  is  electrically  connected  to  one  pair  of  quadrants  as 
is  usually  done,  and  the  scales  of  such  instruments  may  be  so  gradu- 
ated as  to  be  direct  reading. 

An  alternating  current  is  said  to  make  as  many  alternations  per 
minute  as  it  makes  changes  in  direction  in  each  minute.  Instead  of 
speaking  of  the  number  of  alternations  per  minute  of  an  alternating 
current  it  is  quite  common  and  more  scientific  to  speak  of  its  fre- 
quency, that  is,  the  number  of  double  alternations  made  per  seayid. 
The  alternating  current  dynamos  which  have  been  generally  used  in 
this  country  furnish  currents  making  from  15,000  to  16,500 
alternations  per  minute  or  having  frequencies  of  from  125  to  i3£-5> 
though  frequencies  only  half  as  great,  and  even  less,  have  come  into 
'use  during  the  past  two  or  three  years.  The  number  of  alternations 
per  minute  is  equal  to  2X60  or  120  times  the  frequency,  since  60  is 
the  number  of  seconds  in  a  minute.  The  fraction  of  a  second  during 
which  an  alternating  current  makes  two  loops  is  called  \\.§  period. 


224 


There  is  one  very  important  point  in  which  alternating  currents 
differ  radically  from  continuous  currents.  The  point  is  so  important 
that  the  balance  of  this  lesson  will  be  taken  to  illustrate  it. 

When  a  continuous  current  is  passed  through  an  incandescent 
lamp,  the  amount  of  power  expended  by  the  passage  of  the  electric 
current  through  the  lamp  filament,  which  is  converted  into  heat  and 
light,  is  equal  to  C  X  E.  In  the  same  way,  when  an  alternating  cur- 
rent is  passed  through  an  incandescent  lamp  the  amount  of  power 
which  is  expended  in  the  lamp  filament  and  converted  into  light  and 
heat  is  also  equal  to  C  X  E,  where  C  and  E  are  the  effective  values  of 
the  current  and  pressure  measured  by  the  proper  alternating  current 
instruments  which  were  explained  in  the  preceding  lesson.  We 
therefore  see  that  an  incandescent  lamp  which  is  intended  to  give 
sixteen  candle-power  at  a  pressure  of,  say,  no  volts,  will  be  equally 
efficient  when  it  is  connected  to  a  constant  pressure  circuit  which 
furnishes  it  continuous  current  at  a  uniform  pressure  of  no  volts,  or 
when  it  is  connected  to  a  circuit  which  furnishes  it  alternating 
current  at  an  effective  pressure  of  no  volts.  If  the  current  flow- 
ing through  the  lamp  when  it  is  connected  to  the  continuous  current 
circuit  be  measured  by  an  accurate  amperemeter  of  any  kind,  and  a 
measurement  also  be  made  when  the  lamp  is  connected  to  the  alter- 
nating current  circuit  by  an  accurate  electrodynamometer,  exactly 
the  same  amount  of  current  will  be  found  to  flow  through  the  lamp 
in  the  two  cases. 

Now,  suppose  we  take  200  feet  of  No.  7  B.  &  S.  gauge  insulated 
copper  wire.  Its  resistance  is  almost  exactly  one-tenth  of  an  ohm  at 
ordinary  temperatures,  and  it  therefore  requires  only  one-tenth  of  a  volt 
to  send  one  ampere  of  continuous  current  through  it.  This  is  true 
whether  the  wire  be  stretched  out  straight,  wound  in  a  simple  coil  or 
wound  around  an  iron  core,  since  the  resistance  of  the  wire  depends 
only  upon  its  length,  cross  section,  and  material  (Lesson  VII,  page 
48),  and  none  of  these  are  altered  by  coiling  or  winding  up  the  wire. 

To  send  one  ampere  of  alternating  current  of,  say,  a  frequency 
of  125  (15,000  alternations  per  minute)  through  this  wire  when  it  is 
stretched  straight  out  requires  a  tenth  of  a  volt  effective  pressure,  or 
the  same  as  in  the  case  of  a  continuous  current.  The  straight  wire 
therefore  acts  in  the  same  way  towards  continuous  and  alternating 
currents,  exactly  as  does  the  incandescent  lamp  filament,  which  indeed, 
is  nothing  more  than  a  bent  wire  made  of  carbon. 

Now,  if  the  wire  be  coiled  tip,  a  greater  pressure  than  one-tenth  of 
a  volt  is  required  to  send  one  ampere  through  the  wire,  while  if  it.  be 
wound  on  a  big  laminated  iron  core  there  may  be  as  much  as  100  volts, 
or  even  more,  required  to  send  an  ampere  through  the  wire.  We  know 
that  the  resistance  of  the  wire  is  not  changed  by  coiling  it  up  or  by  wind- 
ing it  around  an  iron  core,  so  that  the  actual  resistance  is  one-tenth  of 
p"  ^hm  all  the  time.  This  is  proved  by  the  fact  that  coiling  the  wire 


225 


and  winding  it  around  an  iron  core  does  not  change  the  amount  of 
pressure  required  to  send  one  ampere  of  continuous  current  through 
it.  It  also  may  readily  be  proved  by  measuring  the  resistance  of  the 
wire  by  a  Wheatstone's  bridge  when  the  wire  is  stretched  straight 
out  and  when  it  is  wound  on  an  iron  core. 

The  action  of  the  alternating  current  as  thus  seen  might  lead  us 
to  suppose  that  the  flow  of  alternating  currents  did  not  follow  Ohm's 
law  (Lesson  VII,  page  42).  The  flow  of  alternating  currents  does  fol- 
low Ohm's  law,  however,  and  the  peculiar  action  described  above  is 
easily  explained  as  follows: 

In  Lesson  XXV,  page  217,  it  was  stated  that  on  account  of 
self-induction,  either  an  increase  or  decrease  of  current  in  a  coil  is 
retarded  by  the  magnetic  effect  of  the  different  turns  of  the  coil  tend- 
ing to  stop  any  change  in  the  current.  This  effect  is  magnified  to  a 
large  degree  when  the  coil  is  wound  on  an  iron  core,  since  the  iron 
largely  increases  the  magnetic  effect  of  the  turns  and  therefore  the 
self-induction  of  the  coil ;  while  a  wire  stretched  out  straight  or  bent 
in  a  hairpin,  like  an  incandescent  lamp  filament,  has  very  little  self- 
induction. 

When  a  battery  is  connected  so  as  to  send  a  current  through  a 
straight  wire  the  current  rises  to  its  full  value,  according  to  Ohm's 
law,  almost  instantly.  When  the  same  wire  is  coiled  up  and  con- 
nected to  the  battery,  the  current  does  not  rise  to  its  full  value 
instantly  on  account  of  the  retarding  effect  of  self-induction,  but  the 
delay  is  only  a  very  small  fraction  of  a  second.  Now,  when  the  wire 
is  wound  on  an  iron  core  and  then  connected  to  the  battery,  the  effect  of 
self-induction  is  so  great  that  it  takes  quite  an  appreciable  portion  of  a 
second  for  the  current  to  rise  to  its  full  steady  value.  The  final  steady 
value  reached  by  the  current  is  not  changed  by  the  self-induction^  but  is 
iust  the  same  in  each  case,  IF  THE  PRESSURE  BE  UNIFORM,  because  the 
self-induction  can  have  an  effect  only  while  the  current  is  changing  in 
value. 

An  alternating  current  changes  all  the  time  so  that  it  never  has 
a  steady  value,  and  the  effect  of  self-induction  is  therefore  felt  by  it  all 
the  time.  While  the  current  is  rising,  self-induction  tends  to  hold  it 
back  or  keep  it  from  rising,  and  when  the  current  is  falling,  self-induc- 
tion still  tends  to  keep  it  from  changing.  The  result  is  that  in  a  cir- 
cuit having  self-induction  an  alternating  cnrrent  is  always  retarded  a 
certain  amount  behind  the  alternating  pressure  which  sets  it  up.  This 
same  retardation  causes  the  maximum  value  of  the  current  to  be 
smaller  than  it  would  be  were  there  no  effect  of  self -induction* 

We  therefore  see  that  when  an  alternating  current  flows  through 
a  circuit  which  has  such  a  form  that  its  self-induction  is  appreciable, 
the  alternations  made  by  the  current  corne  a  small  fraction  of  time 
later  than  those  made  by  the  electric  pressure,  and  the  value  of  the 
current  is  smaller  than  if  no  self-induction  were  present.  The  effect 


226 


is  exactly  as  though  the  current  loops  of  Fig.  255  were  not  placed 
directly  under  the  pressure  loops,  but  were  pushed  a  certain  small 
amount  back  of  the  position  of  the  pressure  loops.  The  effect  of  self- 
induction  in  decreasing  the  amount  of  alternating  current  which 
flows  in  a  circuit  depends  upon  the  magnetic  effect  which  the  differ- 
ent parts  of  the  circuit  have  on  each  other,  and  also  upon  the  "fre- 
quency "  of  the  current.  The  same  result  is  brought  about  as  would 
be  given  by  increasing  the  resistance  of  the  circuit  a  certain  amount. 
It  is  therefore  usual  to  speak  of  the  apparent  resistance  of  a  circuit 
through  which  an  alternating  current  flows.  The  effective  current  in  a?i 
alternating  current  circuit  is  then  equal  to  the  effective  electrical  pressure 
applied  to  the  circuit  divided  by  the  apparent  resistance  of  the  circuit. 
This  may  be  called  the  Ohm's  law  of  the  alternating  current  circuit. 
The  apparent  resistance  is  equal  to  the  true  resistance  of  the  wire  compos- 
ing the  circuit  plus  the  effect  due  to  self-induction.  The  true  resistance 
of  the  wire  only  depends  upon  its  length,  cross  section  and  material, 
while  the  effect  of  self-induction  depends  upon  the  magnetic  effect  which 
the  different  parts  of  the  circuit  exert  on  each  other  and  upon  the  fre- 
qttency  of  the  current.  When  a  continuous  current  flows  through  a 
circuit,  the  true  resistance  of  a  circuit,  as  measured  by  a  Wheatstone's 
bridge,  only  need  be  considered,  but  when  an  alternating  current  flows 
through  the  same  circuit,  the  apparent  resistance  comes  into  the 
account. 

The  remarkable  results  which  are  brought  about  in  alternating 
current  circuits  on  account  of  the  current  hanging  back  or  lagging 
behind  the  electrical  pressure  will  be  taken  up  in  the  next  lesson. 
Before  entering  upon  the  next  lesson,  each  member  of  the  classes 
should  study  this  lesson  until  he  gets  a  true  idea  of  the  lagging  of 
the  loops  of  an  alternating  current  behind  the  pressure  which  sets  up 
the  current,  and  the  cause  of  this  lagging. 

Copyrighted,  1895, 


327 


The  National  School  of  Electricity. 

REVIEW  OF  LESSON  XXVI. 

Points  for  Review.     1.     What  is  an  alternator? 

2.  Why  is  the  electrolytic  effect  of  a  pulsating  current  the  same  as  that  of  a  contin- 
uous current  equal  to  its  mean  value? 

3.  Why  does  an  alternating  current  produce  no  electrolytic  effect? 

4.  Why  is  the  heating  effect  of  a  pulsating,  or  of  an  alternating  current,  greater 
than  that  of  a  continuous  current  equal  to  its  mean  value? 

5.  What  is  the  effective  value  of  an  alternating  current? 

6.  Why  is  it  desirable  to  make  alternating  current  measurements  in  effective  values? 

7.  Why  are  instruments  based  on  the  principle  of  the  electrodynamometer,  or  on 
the  hot  wire  principle,  always  used  in  measuring  alternating  currents?' 

8.  Why  are  instruments  based  upon   the  same  principles,  or  on  the  electrostatic 
principle,  always  used  in  measuring  alternating  voltages? 

9.  What  is  the  "  frequency"  of  an  alternating  current?     What  is  its  "period  "  ? 

10.  If  an  alternating  current  has  a  frequency  of  100,  how  many  alternations  does  it 
make  per  minute? 

11.  What  frequencies  are  commonly  used  in  this  country? 

12.  Why  is  the  apparent  resistance  which  a  coil  of  wire  offers  to  the  passage  of 
an   alternating  current  greater  than  the  resistance  which  it  offers  to  the  passage  of  a 
continuous  current? 

13.  Why  is  the  resistance  which  a  straight  wire  offers  to  the  passage  of  an  alter- 
nating current  equal  to  that  which  it  offers  to  the  passage  of  a  continuous  current? 

14.  Why  is  the  apparent  resistance  of  a  coil,  in  which  an  alternating  current  flows, 
increased  by  putting  an  iron  core  in  it? 

15.  Why  does  the  apparent  resistance  of  a  coil  change  when  the  frequency  of  the 
alternating  current  which  flows  through  it  changes? 


LESSON         XXVII. 

ALTERNATING  CURRENTS  AND  ALTERNATING  CUR- 
RENT MACHINERY. 

(Concluded^ 

There  is  an  additional  difference  between  the  effects  of  alter- 
nating currents  and  of  continuous  currents,  when  the  alternating  cur- 
rent in  a  circuit  with  self-induction  lags  behind  the  alternating  pres- 
sure which  causes  it  to  flow.  When  an  alternating  current  flows 
through  a  circuit  which  does  not  have  self-induction,  the  current 
loops  and  pressure  loops  come  together  as  shown  in  Fig.  255.  Then 
we  can  measure  the  power  which  is  used  in  the  circuit  by  an  alter- 
nating current  voltmeter  and  an  electrodynamometer,  because  these 
instruments  measure  the  effective  pressure  and  the  effective  current,  and 
the  two  readings  multiplied  together  give  the  power  used  in  the  circuit 
(Lesson  XXVI,  page  223).  We  can  therefore  measure  the  power  used 

228 


in  an  incandescent  lamp  ivhich  is  operated  on  an  alternating  current 
circuit  by  means  of  an  alternating  current  amperemeter  and  nn  alter- 
nating current  voltmeter,  exactly  in  the  same  way  that  we  would 
measure  the  power  used  by  it  when  operated  on  a  continuous  current 
circuit  (Lesson  XI,  page  83). 

If  a  coil  of  wire,  having  an  iron  core,  be  substituted  for  the  in- 
candescent lamp,  the  current  loops  are  caused  by  the  effect  of  self- 
induction  to  lag  behind  the  pressure  loops  (Lesson  XXVI,  page  226). 
When  this  is  the  case  we  are  not  able  to  measure  the  power  used  in 
the  coil  by  an  amperemeter  and  a  voltmeter,  as  we  did  in  the  case  of 
an  incandescent  lamp,  because  in  this  case  the  product  of  the  effective 
current  and  the  effective  pressure  is  not  equal  to  the  power.  The 
actual  power  used  in  the  circuit  is  less  than  the  value  given  by  the 
product  of  the  effective  current  and  pressure.  At  EACH  INSTANT  the 
power  consumed  in  the  circuit  is  equal  to  the  product  of  the  current 
and  the  pressure  at  that  instant,  exactly  as  is  the  case  when  the  cur- 
rent and  pressure  loops  are  together  (Lesson  XXVI,  page  223),  but  when 
the  current  lags  behind  the  pressure,  the  total  power  consumed  is  less 
than  would  have  been  used  in  sending  the  same  current  under  the  same 
pressure  through  a  circuit  without  self-induction.  The  moral  of  this 
is  not  to  try  to  measure  the  power  used  in  any  alternating  current 
circuit  having  self-induction,  by  an  amperemeter  and  a  voltmeter. 
For  instance,  if  the  alternating  current  flowing  in  the  primary  coil 
of  a  transformer  be  measured  and  its  value  be  multiplied  by  the  alter- 
nating pressure  which  causes  the  current  to  flow",  the  product  does 
not  represent  the  power  used  by  the  transformer. 

The  power  used  when  an  alternating  current  is  caused  to  flow 
through  a  circuit  which  has  self-induction  may  be  measured  by  a 
proper  wattmeter;  such,  for  instance,  as  that  made  out  of  an  electro- 
dynamometer,  explained  on  page  85  of  Lesson  XII.  The  indications 
of  such  a  wattmeter  when  connected  to  the  circuit  as  directed  in  Les- 
son XII  are  directly  proportional  to  the  power  used  in  the  circuit, 
because  they  are  the  AVERAGE  of  the  values  of  the  power  given  to  the 
circuit  at  every  instant.  If  it  is  desired  to  find  out  how  much  power 
is  wasted  in  the  iron  core  of  an  alternating  current  transformer,  for 
instance,  it  can  be  quickly  done  by  connecting  up  a  wattmeter  as 
shown  in  Fig.  258,  for  then,  if  the  wattmeter  has  been  calibrated,  its 
readings  will  at  once  give  the  power. 

Alternating  currents  are  widely  used  for  the  distribution  of 
electric  currents  for  the  purpose  of  electric  lighting,  because  it  is 
possible  to  use  a  high  pressure  on  the  distributing  lines  and  thus 
make  a  saving  in  the  expense  of  wires  (Lesson  XX II,  page  135),  and 
the  high  pressure  may  be  reduced  with  little  loss  of  power  by  means 
of  induction  coils  or  transformers  to  a  pressure  which  it  is  safe  to  use 
in  houses  (Lesson  XXV,  page  216).  These  transformers  consist 


of  two  coils,  the  primary  and  secondary  coils,  which  have  well 
laminated  iron  cores  made  of  strips  or  '  'stampings' '  of  thin  wrought 
iron  laid  together  in  such  a  manner  that  they  make  a  core  for  the 
coils  and  also  enclose  them  so  as  to  make  a  complete  magnetic  circuit 
for  the  magnetism  set  up  by  a  current  in  the  coils.  The  primary  coil 
usually  consists  of  many  turns  of  small  wire,  while  the  secondary 
coil  consists  of  fewer  turns  of  larger  wire.  The  coils  are  wound  on  a 
"  former"  and  are  carefully  insulated  with  mica,  rubber  insulating  tape, 
or  other  insulating  materials,  and  the  core  is  then  built  up  by  slip- 
ping the  stampings  into  position.  The  right-hand  cut  of  Fig.  248 
shows  the  two  coils  of  a  transformer  with  the  stampings  which  form 
the  core.  The  left-hand  cut  of  the  same  figure  shows  the  transformer 
after  it  has  been  placed  in  a  water  tight  iron  case,  as  is  usually  done 
to  protect  it  from  injury  when  hung  against  the  wall  of  a  house  or 
on  the  pole  of  an  electric  light  company.  In  the  particular  type  of 
transformer  shown  in  this  figure,  it  is  usual  to  fill  up  the  case  with  a 
heavy  paraffine  oil  which  improves  the  insulation  of  the  coils  from 
each  other.  In  Fig.  259  is  shown  the  coils  of  a  transformer  made  by 
another  maker  and  also  the  complete  transformer  out  of  its  case, 
while  Fig.  260  shows  the  same  transformer  in  its  case.  Figs.  261 
and  262  show  other  transformers.  In  the  latter  cut,  the  transformer 
is  shown  as  it  hangs  on  the  side  of  a  house  in  winter  and  the  need 
of  the  protecting  case  is  made  evident.  Each  of  the  transformers  thus 
shown  is  made  by  different  manufacturers,  but  their  similarity  in 
construction  is  evident  at  a  glance.  There  are  some  differences  in 
the  number  and  shape  of  the  iron  plates  used  in  the  cores,  the  sizes 
of  wire  and  number  of  turns  composing  the  coils,  etc.,  but  the  greatest 
differences  apparent  to  the  sight  are  differences  in  the  shapes  of  the  iron 
cases.  In  fact,  the  real  differences  between  the  transformers  are  very 
small,  but  even  these  small  differences  affect  their  usefulness  very  much. 
The  iron  core  of  a  transformer  is  magnetized  first  in  one  direction  and 
then  in  the  other,  by  the  alternating  currents  in  the  coils,  and  as 
the  magnetic  molecules  are  reversed,  there  is  a  loss  of  power  caused  by 
hysteresis  (Lesson  XX,  page  157).  There  is  also  a  loss  of  power  caused 
by  eddy  or  foucault  currents  which  are  set  up  in  the  iron  core.  These 
losses  are  quite  small  compared  with  the  full  load  of  the  transformer 
(from  3  to  6  per  cent),  but  when  a  great  many  lightly  loaded  trans- 
formers are  operated  all  day  long,  as  is  done  in  many  electric  light 
plants,  the  total  power  lost  may  cause  a  great  expense.  The  losses 
in  the  cores  of  transformers  should  therefore  always  be  tested  by  elec- 
tric light  companies  before  putting  the  transformers  into  service,  and 
if,  the  losses  are  larger  than  they  ought  to  be,  the  transformers  should 
be  sent  back  to  the  makers.  The  tests  can  be  made  by  connecting 
up  a  wattmeter  to  a  transformer  as  shown  in  Fig.  258.  If  the  second- 
ary circuit  is  left  open,  the  reading  of  the  wattmeter  shows  the  loss 


230 


100  'VOLT 

CURRENT  SUPPI.V 


T  METER 


PRE.S!  URE  COIL 


CURRENT    COIL- 


TRANSFORMGR 


-•  VOLTS^- 


CIRCUIT  OPEN 

FIG.  258. 


FIG.  259. 


FIG,  263. 


FIG.  264. 


231 


FIG.  262. 


FIG.  265. 


FIG.  268. 


FIG.  266. 

232 


FIG.  267. 


FIG.  271. 


234 


of  power  caused  by  hysteresis  and  foucault  currents.  The  following 
table  shows  approximately  the  amount  of  power  which  is  lost  in 
transformers  of  the  best  makes. 

CAPACITY  OF  TRANSFORMERS.  LOSS  IN  CORE. 

500  watts  =  10  lights.  25  watts 

1000      "     =  20       "  40      u 

1500      "     =  30       "  50      « 

2500      4'     =  50       "  60      u 

4500      "       =  90       u  80      " 

In  nearly  all  electric  lighting  plants  where  alternating  currents 
are  used  in  this  country,  the  pressure  generated  by  the  alternator  is 
between  1000  and  1200  volts,  while  the  pressure  desired  at  the  lamps 
is  between  100  and  no  volts,  or  50  and  55  volts.  The  transformer 
coils  must  be  wound  so  that  the  number  of  primary  turns  has  the 
same  relation  to  the  number  of  secondary  turns  as  the  primary 
pressure  has  to  the  desired  secondary  pressure.  If  the  pressure  is 
reduced  from  1000  volts  to  100  volts  there  must  be  one  tenth  as  many 
turns  in  the  secondary  as  in  the  primary,  and  if  the  pressure  is 
reduced  to  50  volts  the  secondary  must  have  one  twentieth  as  many 
turns  as  the  primary.  Since  the  power  given  out  by  a  transformer 
is  nearly  as  great  as  that  given  to  it,  the  current  in  the  secondary 
coil  is  nearly  as  much  greater  than  the  primary  current  as  the 
secondary  pressure  is  smaller  than  the  primary  pressure. 

We  have  in  transformers  a  most  striking  and  wonderful  example 
of  the  transfer  of  power  from  one  electrical  circuit  (the  primary  cir- 
cuit) to  another  circuit  (the  secondary)  without  the  circuits  being  in  any 
way  electrically  connected  with  each  other.  The  inductive  action  goes 
on  just  as  well  if  the  two  coils  of  the  transformer  are  separated  by 
glass  or  mica  as  if  they  are  wound  close  together.  It  is  only  necessary 
for  the  magnetic  circuit  to  be  properly  arranged  so  that  the  magnetism 
which  is  set  up  by  the  primary  coil  shall  all  pass  through  the  secondary 
coil.  The  action  of  transformers  is  really  no  more  wonderful  than 
the  action  of  dynamos,  but  it  has  the  striking  peculiarity  that  no 
mechanical  motion  is  concerned  in  the  transformations. 

As  already  said,  alternating  current  dynamos,  or  alternators,  are 
built  upon  the  same  principles  as  continuous  current  dynamos,  but 
the  armature  is  wound  in  coils  which  are  connected  in  series,  and  the 
two  ends  are  brought  to  separate  collecting  rings.  The  field  magnet 
usually  has  as  many  poles  as  there  are  coils  on  the  armature,  and  the 
number  of  alternations  of  the  current  per  minute  is  equal  to  the  number 
of  poles  in  the  field  magnet  multiplied  by  the  number  of  revolutions  made 
by  the  armature  per  minute.  Fig.  263  shows  a  diagram  of  the  connec- 
tions of  an  alternator  armature.  The  coils  marked  AAA  are  armature 
coils  and  the  rings  marked  CC  are  the  collecting  rings  on  which  the 


235 


brushes  BB  rub.  The  arrows  show  the  way  the  current  flows 
through  the  armature.  Fig.  264  sL.cws  the  way  the  magnet  poles 
are  arranged  for  an  alternator  having  an  armature  of  the  form  shown 
in  the  above  diagram.  This  arrangement  of  the  armature  and  fields, 
which  is  quite  common  in  foreign  alternators,  is  illustrated  in  Fig. 
265  which  shows  the  form  of  alternator  built  by  an  English 
maker. 

In  this  country  the  coils  are  usually  laid  on  the  surface  of  a 
laminated  drum  core,  or  in  grooves  cut  in  such  a  core.  Fig.  266 
shows  a  finished  alternator  armature  the  coils  of  which  are  laid  on 
the  surface  of  the  core.  A  layer  of  insulation  is  put  on  over  the 
coils,  and  over  this  wire  bands  are  placed  to  hold  the  coils  in  place. 
This  figure  also  shows  on  the  same  shaft,  the  armature  of  a  small 
continuous  current  dynamo  which  is  used  to  magnetize  the  field 
magnets  of  the  alternator.  Fig.  267  shows  the  way  in  which  coils 
are  sometimes  fixed  in  grooves  cut  in  the  armature  core. 

Since  no  commutator  is  required  with  an  alternator,  it  is  not 
necessary  for  the  armature  to  revolve,  and  the  field  may  be  revolved 
instead.  In  this  case,  the  magnetizing  current  is  carried  to  the  field 
windings  through  collector  rings,  and  the  armature  terminals  are 
connected  directly  to  the  circuit.  It  is  also  possible  to  build  alter- 
nators in  which  neither  the  field  nor  armature  revolves,  but  in  which 
keepers  of  iron  are  moved  so  as  to  make  and  break  the  magnetic 
circuit  of  the  field  magnets  and  thus  cause  currents  to  be  induced  in 
the  stationary  armature.  The  field  magnets  of  alternators  must 
always  be  excited  by  a  continuous  current,  which  is  usually  generated 
by  a  separate  small  continuous  current  dynamo  called  an  exciter. 
An  exciter  is  shown  alongside  of  the  alternators  in  Figs.  265,  268, 
269  and  270. 

The  general  forms  of  alternators  constructed  by  the  different 
American  companies  are  quite  similar  to  each  other,  as  shown  in  Figs. 
268,  269  and  270.  Fig.  271  shows  one  of  the  great  1,000  horse-power 
alternators  which  were  used  in  1893  to  light  the  buildings  at  the 
World's  Fair.  These  machines,  which  are  the  largest  alternators  ever 
built,  are  now  used  in  various  electric  light  stations. 

Alternators  cannot  be  worked  in  parallel  with  each  other  with 
the  ease  which  is  possible  with  continuous  current  dynamos.  If  two 
similar  continuous  current  dynamos  are  to  be  connected  in  parallel 
they  are  simply  brought  to  their  usual  speeds,  and  their  field  mag- 
netization is  adjusted  until  the  two  machines  produce  the  same 
pressure.  They  may  then  be  connected  in  parallel  and  will  work 
together  very  well.  When  two  alternators  are  to  be  connected  in 
parallel,  it  is  necessary  not  only  to  make  their  pressures  equal,  but  to 
bring  them  to  exactly  equal  frequencies  or  to  synchronism,  and  also  to 
arrange  them  so  that  the  current  loops  given  by  the  two  machines  are 


•?S6 


in  exact  ttnison  or  step.  On  account  of  the  difficulty  in  the  way  of 
properly  synchronising  and  stepping  alternators  they  are  not  usually 
operated  in  parallel  in  this  country,  though  it  is  quite  commonly 
done  in  foreign,  countries. 

If  an  ordinary  alternator  is  brought  to  synchronism  with  another 
machine  it  may  be  run  by  the  latter  as  a  motor,  but  it  will  not  start 
itself  as  would  a  continuous  current  motor,  nor  is  it  possible  to 
excite  the  field  magnets  of  the  motor  from  the  alternating  current 
circuits.  It  is  therefore  not  convenient  to  use  such  machines,  called 
synchronous  motors  for  common  purposes  (though  they  are  sometimes 
used  for  special  purposes),  and  other  methods  of  operating  alternating 
current  motors  are  being  sought  after.  The  most  promising  of  these 
methods  is  coming  into  considerable  use.  It  consists  of  combining 
the  effects  of  two  or  three  separate  alternating  currents  in  what  are 
known  as  two-phase  or  three-phase  systems. 

A  second  set  of  windings  may  be  placed  on  an  alternator  arma- 
ture with  the  centers  of  its  coils  half-way  between  the  first  set  (as, 
for  instance  if  another  winding  were  put  on  the  armature  shown  in 
diagram  in  Fig.  263,  with  its  coils  between  those  shown  in  the  Fig.), 
then  the  currents  generated  in  the  second  set  of  coils  will  have  their 
maximum  points  just  one  quarter  of  a  period  after  the  current  in 
the  first  winding.  That  is,  the  two  currents  will  have  a  difference 
of  phase  equal  to  quarter  of  a  period  or  90°.  The  relation  of  the 


A 

FIG.  A. 


two  currents  to  each  other  is  shown  in  Fig.  A  where  the  curves 
A  and  B  represent  the  two  currents.  These  two  currents  may  be 
used  separately  or  they  may  be  used  together  as  a  two-phase  system, 
the  two  currents  being  carried  in  separate  circuits  composed  of  three 
wires  very  much  as  the  three  wires  compose  the  circuits  of  the  three 
wire  system  for  continuous  current  distribution.  Instead  of  two 
windings  three  separate  windings  may  be  placed  on  the  armature 
in  such  a  way  that  the  three  currents  produced  in  them  differ  from 
each  other  in  phase  by  one  third  of  a  period  or  1 20°.  The  relations 
of  these  currents  are  illustrated  in  Fig.  B.  These  currents  may  be 
used  separately  or  they  may  be  used  together  as  a  three-phase  system, 
the  three  currents  being  carried  in  separate  circuits  composed  of 

237 


three  wires,  the  circuits  being  made  up  of  the  three  wires  taken  in 
*iairs.  Thus,  if  the  three  dots  in  Fig.  C  represent  the  cross  section 
of  the  three  wires,  then  current  A  is  carried 
in  the  circuit  composed  of  the  wires  a  and 
b.  Current  B  is  carried  in  the  circuit  com- 
posed of  the  wires  b  and  c,  and  current  C  is 
carried  in  the  circuit  composed  of  the  wires 
c  and  a.  Either  a  two-phase  or  three-phase 
alternator  which  is  arranged  to  furnish  cur- 
rents to  three  wires  requires  only  three  collect- 
ing rings,  though  if  the  currents  were  to  be 
used  separately  four  and  six  rings  would  be 
respectively  required.  Two-phase  and  three- 
phase  systems  are  frequently  called  polyphase  or  multiphase  (many 
current)  systems,  and  the  motors  which  are  ordinarily  operated  on 
polyphase  systems  are  called  induction  motors. 


The  action  of  induction  motors  may  be  explained  by  reference 
to  Fig.  D  which  is  an  illustrative  diagram  of  a  three-phase  motor. 
The  field  magnet  of  the  motor  is  a  ring  which  is  wound  with  three 
separate  coils,  P,  Q  and  R,  each  of  which  is  supplied  with  one  of 
the  currents  of  the  three-phase  system  through  the  wires  a,  b  and  c. 
Since  the  maximum  values  oi  the  three  currents  which  thus  flow 
through  the  coils  P,  Q  and  R  follow  one  another  with  a  phase 
difference  of  a  third  of  a  period  their  maximum  points  appear  to 
chase  each  other  around  the  ring.  The  magnetic  effect  of  each 
coil  at  every  instant  is  proportional  to  the  current  flowing  in  it,  and 
the  combined  effect  of  the  three  currents  sets  up  a  magnetic  field 
which  rotates  around  the  ring  with  the  maximum  value  of  the  cur- 
rents. The  space  inside  of  the  field  ring  is  occupied  by  an  armature 


238 


consisting  of  a  grooved  drum  built  up  out  of  iron  discs.  Insulated 
copper  rods  are  laid  in  the  grooves,  and  the  rods  are  either  all 
connected  together  by  end  rings  as  in  Fig.  E  or  they  are  connected 
in  sets  as  indicated  in  Fig.  D.  The  rotating  magnetic  field  set  up  by 
the  currents  in  the  windings  P,  Q  and  R,  induces  currents  in  the 
armature  conductors  and  these  in  turn,  on  account  of  the  reactions 
between  currents  and  a  magnetic  field  explained  in  Lesson  VI. ,  cause 
the  armature  to  revolve  as  nearly  as  possible  in  synchronism  with 
the  rotating  field.  The  armature  with  the  bars  all  connected 
together  as  shown  on  Fig.  E  is  called  a  squirrel  cage  armature. 


FIG.  F. 


FIG.  H. 


MAIN  WINDING 


PRIMARY 


IMMJ 

rr 


FIG.  I. 


SECONDARY 
FIG    J. 


The  coils  of  polyphase  machinery  may  be  connected  in  three  dii 
ferent  combinations,  two  of  which  are  called  respectively  the  mesh 
and  star  connections  and  are  represented  in  diagram  by  the  Figs.  F 
and  G.  The  third  method  of  connecting  the  coils  is  a  combination 
of  the  mesh  and  star  methods.  The  coils  on  the  fields  of  Fig.  D  are 
connected  in  the  mesh  method  and  Fig.  H  shows  in  diagram  a  two- 
phase  motor  which  may  be  operated  with  its  field  coils  mesh  con- 
nected to  a  three-wire  two-phase  system  or  with  the  coils  connected 
separately  to  two  separate  circuits  which  carry  alternating  currents 
with  90°  difference  of  phase.  Two-phase  machinery  is  constructed 
in  this  country  by  the  Westinghouse  Co.  and  the  Stanley  Electric 
Co.,  while  three-phase  machinery  is  constructed  by  the  General 
Electric  Co. 

A  special  type  of  three-phase  alternator  (called  a  monocyclic 
alternator)  is  constructed  by  the  General  Electric  Co.  for  use  where 
electric  lighting  is  the  principal  object  but  it  is  desired  to  operate 
some  motors  from  the  lighting  circuit.  The  armature  of  a  mono- 
cyclic  alternator  carries  a  main  coil,  the  two  ends  of  which  go  to 
collector  rings  and  an  auxiliary  (or  teaser)  coil,  one  end  of  which 
goes  to  a  third  collector  ring  and  the  other  end  of  which  is  connected 
to  the  middle  of  the  main  coil,  Fig.  i.  Transformers  to  be  used  for 
electric  lightning  are  connected  in  the  usual  way  between  the  wires 
running  from  the  brushes  on  the  main  collector  rings,  but  a  three- 
phase  current  may  be  obtained  by  connecting  two  transformers 
between  the  main  and  teaser  wires  as  shown  in  Fig.  J.  The  three 
circuits  of  the  three-phase  currents  are  between  the  wires  A  B,  B  C 
&  A  C.  If  the  three  terminals  of  a  three-phase  induction  motor 
are  connected  to  these  wires  it  will  run  exactly  as  though  connected 
to  a  regular  three-phase  circuit. 

Polyphase  alternators  may  be  used  as  synchronous  polyphase 
motors  under  conditions  similar  to  those  already  explained  for  single 
pliase  machine. 

Copyrighted,  1895, 


The  National  School  of  Electricity. 

REVIEW    OF    LESSON       XXVII. 

Points  for  review.      1.  How  is  the  power  which  is  used  in  any  alternating  circuit 
measured? 

2.  Why  are  alternating  currents  used  for  electric  lighting? 

3.  What  are  transformers  and  how  are  they  made? 

4.  Why  are  transformers  put  in  an  iron  case? 

5.  Why  should  transformers  be  tested  by  electric  light  companies  before  they  are  put 
into  use? 

6.  How  can  this  testing  be  done? 

7.  How  are  alternators  built? 

8.  How  can  the  number  of  alternations  per  minute  made  by  the  current  produced 
by  an  alternator  be  calculated? 

9.  If  an  alternator  has  ten  poles  and   the  armature   makes  1,500  revolutions  per 
minute,  how  many  alternations  per  minute  are  made  by  the  current  which  is  produced? 

10.  What  is  the  frequency  of  the  current? 

11.  What  is  an  exciter? 

12.  What  must  be  done  to  make  alternators  run  in  parallel? 

13.  What  are  synchronous  motors? 

14.  Why  can  they  not  be  commonly  used? 

15.  What  is  meant  by  the  words  polyphase  and  multiphase? 

16.  What  are  two-phase  and  three-phase  alternating  current  systems? 

17.  How  many  wires  are  necessary  in  two-phase  and  three-phase  systems? 

18.  What  are  induction  motors,  and  how  do  they  work? 

19.  What  is  a  squirrel  cage  armature? 

20.  What  is  meant  by  mesh  connection?     By  star  connection? 


LESSON        XXVIII. 

MISCELLANEOUS  APPLICATIONS  OF  ELECTRIC 

MOTORS. 

During  five  or  six  years  past  electric  motors  have  come  to  be  al- 
most a  necessity  to  people  living  in  small  cities  who  use  small 
amounts  of  power.  The  wonderful  way  in  which  electric  motors  have 
come  into  general  use  is  very  striking.  The  number  of  electric 
motors  used  in  Chicago  in  the  year  1889  was  very  small,  while  in 
1894  motors  to  more  than  four  thousand  horse  power  capacity  were 
supplied  with  current  from  the  distribution  system  of  the  Edison 
Illuminating  Company  of  that  city.  In  addition  to  these  motors 
many  more  are  supplied  with  current  from  other  central  or  isolated 
plants.  Chicago  is  not  at  all  exceptional  in  the  number  of  electric 
motors  which  its  inhabitants  use,  for  large  numbers  are  also  used  in 

241 


New  York,  Boston,  Philadelphia  and  other  large  cities.  In  fact 
electric  motors  are  as  necessary  to  the  small  users  of  power  who  live 
in  American  cities  as  gas  engines  are  to  the  citizens  of  Paris,  and  they 
have  also  become  household  necessities  in  many  places.  The  use  of 
electric  motors  in  small  shops  and  for  household  purposes  is  by  no 
means  limited  to  the  large  cities,  but  in  all  places  where  a  continuous 
current  supply  is  at  hand  throughout  the  day,  electric  motors  are 
found  in  many  kinds  of  service.  They  are  also  connected  with  many 
isolated  plants.  One  of  their  commonest  uses  is  to  drive  small  fans 
for  stirring  up  the  air  in  a  room  in  the  hot  summer  days.  Such /2m 
motors  are  very  common  in  offices,  theaters  and  public  places.  An 
interesting  use  of  fan  motors  is  made  on  the  electrically-lighted  trains 
of  the  Pennsylvania  Railroad  and  the  -Chicago,  Milwaukee  and  St. 
Paul  Railroad,  the  dining  cars  of  which  are  made  very  comfortable 
on  hot  summer  evenings  by  several  fan  motors,  which  take  current 
from  the  electric  light  circuits.  An  example  of  motors  used  with  an 
isolated  plant  is  to  be  seen  in  the  great  plant  of  the  Auditorium  Hotel 
and  Theater  in  Chicago,  where  motors  having  a  combined  capacity 
of  several  hundred  horse  power  are  in  daily  use.  These  motors  are 
used  to  drive  ventilating  fans  and  small  blowers  as  shown  in  Figs. 
272  and  273,  to  run  coal  and  ash  hoisters,  meat  choppers  and  coffee 
grinders  in  the  kitchen,  machinists'  tools  for  the  repair  shop,  bel- 
lows for  the  great  organ  (Fig.  274),  to  drive  a  small  dynamo  which 
furnishes  current  for  the  hotel  bells  and  for  other  purposes.  Some 
of  the  dynamos  of  this  plant  are  required  to  run  all  day  and  all  night, 
so  that  a  supply  of  current  is  always  on  hand  by  means  of  which  the 
motors  may  be  operated. 

The  uses  to  which  electric  motors  may  be  put  are  almost  end- 
less, but  a  few  of  the  common  applications  are  illustrated  in  the 
figures  of  this  lesson.  In  Figs.  275  and  276  are  shown  a  sewing- 
machine  and  a  dentist's  lathe,  each  with  a  motor  connected  to  it. 
Figs.  277  to  281  show  various  purposes  for  which  pumps  driven  by 
electric  motors  are  used.  Electric  motors  driving  con  tractors'  hoists, 
which -are  used  in  the  construction  of  large  buildings,  are  shown  in 
Figs.  282  and  283,  and  an  electric  mining  hoist  is  shown  in  Fig.  284. 
Fig.  285  shows  a  115  horse  power  Edison  motor  driving  line  shafting 
in  Machinery  Hall  at  the  World's  Columbian  Exposition.  In  Fig. 
286  is  shown  a  flour  mill,  which  is  driven  by  the  electric  motor  which 
appears  in  the  figure.  This  list  of  illustrations  might  be  extended 
to  an  indefinite  extent  without  exhausting  the  various  purposes  for 
which  electric  motors  may  be  used,  and  for  which,  indeed,  they  are 
used  in  great  numbers. 

A  place  in  which  electric  motors  are  coming  to  be  very  much 
appreciated  and  widely  used  is  in  great  manufactories.  The  ordinary 
method  of  carrying  power  through  shops  by  means  of  great  belts  and 


FIG.  274. 


243 


FIG.  276. 


FIG.  275. 


FIG.  277. 


FIG.  283. 


FIG.  279. 


FIG.  281. 


FIG.  280. 


FIG.  282. 


FIG.  284. 


FIG.   285. 

247 


FIG.  286. 


FLEXIBLE   DRILL 


LATHE 


FIG.  290. 


248 


FIG.  287. 


249 


FIG.  288. 


250 


FIG    289. 


251 


OF  THE 

VNIVERSIT 


FIG.  291. 


jr.  ™ar "»'"»:•  •» IK-  »  *  I 

Fi  ii  ii  m  ai  ill 


FIG.  292. 


252 


heavy  shafts  is  very  wasteful  of  power.  Prof.  Flather  says  in  his  book  on 
power  measurements  that  wherever  measurements  have  been  made  in 
even  the  best  arranged  shops,  the  losses  of  power  on  account  of  shafting 
and  belting  are  shown  to  be  enormous.  The  attached  table  shows 
the  amount  of  power  lost  in  belting  and  shafting  and  the  amount 
actually  delivered  where  it  is  required  for  use,  for  every  hundred  horse 
power  developed  by  the  engine.  The  table  shows  that  from  one- 
third  to  three-fourths  of  the  power  of  the  engine  is  actually  wasted  in 
simply  making  shafting  revolve  and  causing  the  belts  and  gears  to  run: 

POWER  LOST,      POWER  USED, 
NAME  OF  WORKS.  pER  ^^  pER 


Union  Iron  Works  ...................  23  77 

Frontier  Iron  &  Brass  Works  ..........  32  68 

Baldwin  Locomotive  Works  ...........  80  20 

Wm.  Sellers  &  Co  ...................  40  60 

Pond  Machine  Tool  Co  ...............  41  59 

Yale  &  Towne  Co  ...................  49  51 

Ferracute  Machine  Co  ................  31  69 

Bridgeport  Forge  Co  .................  50  50 

Shafts  and  belts  are  a  great  nuisance  in  shops,  and  any  conven- 
ient arrangement  which  can  take  their  place  would  be  very  useful, 
even  if  it  did  not  save  power.  A  convenient  arrangement  which 
takes  their  place  and  at  the  same  time  saves  much  power  is  of  the 
greatest,  service.  It  is  in  this  place  that  the  electric  motor  shows  one 
of  its  finest  characteristics.  In  Fig.  287  is  shown  a  large  machine 
shop  in  which  the  power  is  distributed  by  shafts  and  belts,  which 
give  the  shop  somewhat  the  appearance  of  a  forest,  while  in  Fig. 
288  is  shown  a  .similar  shop  after  the  lathes,  planers  and  other  ma- 
chines are  arranged  to  be  driven  by  electric  motors.  The  motors  are 
close  to  the  machines  and  the  electric  wires  leading  to  them  are  put 
out  of  the  way  so  that  the  shop  presents  an  appearance  which  is  much 
improved.  The  improvement  is  as  great  in  fact  as  in  appearance, 
because  the  removal  of  shafting  and  belts  removes  a  great  source  of 
danger  and  inconvenience,  and  electrical  distribution  of  the  power  is 
much  less  wasteful  than  its  distribution  by  shafts  and  belts.  With  a 
properly  arranged  electrical  distribution,  as  much  as  one-half  or  three- 
fourths  of  the  powei  developed  by  the  engine  may  be  delivered  at  the 
point  where  it  will  be  of  use.  Only  from  one-fourth  to  one-half  of 
the  power  of  the  engine  is  wasted  instead  of  a  waste  of  from  one-third 
to  three-fourths  of  the  engine's  power  as  is  the  case  when  the  power 
is  distributed  by  shafts  and  belts.  The  reduction  in  the  proportion  of 
the  power  which  is  wasted  and  lost,  which  may  be  made  by  using 
electricity  instead  of  belts  and  shafts  is  worth  a  great  many  dollars  to 
the  owner  of  the  shops,  and  many  shops  have  therefore  been  arranged 
for  electrical  transmission,  while  many  more  are  being  so  arranged. 


353 


Ilj  Fig.  289  is  shown  the  power  house  where  electricity  is  generated 
to  operate  the  motors  of  one  great  manufacturing  establishment. 
Fig.  290  shows  the  way  in  which  motors  are  applied  to  drive  lathes, 
drills  and  other  machines,  while  Fig.  291  shows  a  motor  which 
drives  an  elevator  gear  and  drum  without  the  intervention  of  belts  01 
pumps. 

The  arrangement  of  electric  motors  which  will  give  the  btst 
results  in  any  shop  depends  upon  a  great  many  things,  and  can  only 
be  arrived  at  by  good  judgment.  The  ideal  method  would  be  to 
have  one  or  more  motors  built  as  a  part  of  every  machine  in  the 
establishment,  but  this  would  make  the  machinery  cost  too  much 
money  and  consequently  cannot  be  carried  out,  though  it  would  prob- 
ably be  the  most  efficient  arrangement  which  it  is  possible  to  make. 
The  next  best  arrangement,  and  the  one  which  is  usually  adopted,  is 
to  have  all  large  machines  which  require  considerable  power  fur- 
nished with  separate  motors.  These  may  be  built  into  the  machines, 
thus  doing  away  with  all  unnecessary  belting  or  gearing,  or  they 
may  be  directly  belted  to  the  usual  driving  pulleys  of  the  machines. 
All  smaller  machinery  may  be  arranged  in  groups  of  two  to  six  ma- 
chines with  a  motor  to  supply  power  to  the  machine  of  each  group 
through  a  light  shaft. 

The  amount  of  power  required  to  drive  different  classes  of  ma- 
chinery is,  as  a  general  rule,  quite  uncertain. 

The  width  of  the  belt  which  is  commonly  used  on  a  machine  is 
some  indication  of  the  power  required,  as  it  may  be  assumed  that  a 
single  leather  belt  when  running  at  the  ordinary  speed  used  in  shops 
will  satisfactorily  drive  from  one  to  two  horse  power  per  inch  of 
width.  A  double  belt  will  generally  drive  about  twice  as  much  as  a 
single  one.  An  exact  estimate  of  the  power  used  by  any  machine 
cannot  be  made  from  the  size  of  its  belt,  however,  since  the  driving 
power  of  a  belt  depends,  amongst  other  conditions,  directly  upon  its 
speed,  and  even  at  ordinary  speeds  it  may  transmit  very  much  more 
power  than  the  rule  given  above  would  indicate,  though  its  operation 
would  be  unsatisfactory. 

The  ordinary  manufacturers  of  machinery  seldom  have  accurate 
information  in  regara  to  the  power  which  is  required  to  drive  the 
machines  which  they  build,  but  the  following  rules,  when  taken  in 
connection  with  the  information  presented  by  the  widths  of  the  driv- 
ing pulleys,  are  useful : 

1.  Fast  running  machinery  takes  more  power  in  proportion  than 
slow  running  machinery. 

2.  Machines  which  are  intended  to  perform  easy  operations  very 
rapidly   may  require   much  more  power  than  machines  which  are 
required  to  perform  much  more  severe  operations  at  a  slower  rate. 
Thus,  wood- working  machinery  usually  requires,  on    account  of  its 
rapid  speed,  considerably  more  power  in  proportion  than  iron-working 
machinery,  though  the  latter  works  a  much  tougher  material. 

3.  In  ordinary  machine  shops  the   power  required  at  the   ma- 


254. 


chines  is  about  one  horse-power  per  thousand  square  feet  of  floor, 
motors  put  in  on  that  basis  will  generally  do  the  work  satisfactorily, 
provided  the  machines  are  properly  grouped  and  the  motors  are  so 
arranged  that  losses  in  belts  and  shafting  are  inappreciable. 

4.  Engine  lathes  and  similar  tools  for  iron  work,  of  sizes  not 
exceeding  a  swing  of  20  inches,  require  from  ^  to  i   horse-power. 
Larger  lathes  may  require  as  much  as  three  horse-power,  but  seldom 
more. 

5.  Planers  and  similar  tools  for  iron  work  require  from  2  to  5 
horse-power,  depending  upon  their  size  and  the  work  they  do. 

6.  Shapers,  milling  machines,  drills,  and  other  smaller  tools  for 
iron  work,  ordinarily  require  less  than  one  horse-power. 

7.  Planers  for  wood  working  require  from  5  to  25  horse-power, 
depending  upon  their  size  and  work. 

8.  Circular  saws  require  from   i    to   10  horse-power,  depending 
upon  their  work. 

9.  Smaller  wood-working  tools  seldom  require  as  much  as  one 
horse-power. 

10.  Large  printing  presses,  such   as  are  used  for  book  printing, 
require  from  2  to  5  horse-power. 

11.  Small  job    printing  presses  require   from    V%    to    y2   horse- 
power. 

12.  Sewing  machines  requirt  irom  2V  to  l/%  horse-power. 

13.  Passenger  elevators  require  from  10  to  40  horse-power. 

14.  Freight  elevators  ordinarily  require  from   2   to   10   horse- 
power. 

15.  By  placing  several  small  machines  which  are  operated  inter- 
mittingly  in  one  group,  the  power  of  the  motor  required  to  drive  the 
group  may  be  much  less  than  would  be  required  to  drive  all  the  ma- 
chines constantly. 

Before  leaving  this  subject,  the  use  of  electricity  on  boats  must 
be  touched  upon.  Fig.  292  shows  one  of  the  "electric  launches" 
which  proved  such  a  success  on  the  lagoons  at  the  World's  Fair,  and 
which  are  now  used  in  Milwaukee  and  other  cities.  These  boats  are 
very  much  like  small  steam  launches  or  naptha  launches,  but  instead 
of  a  hot  steam  boiler  and  engine,  or  a  disagreeable  naptha  engine,  an 
electric  motor  is  attached  to  the  shaft  of  the  screw  propeller.  This 
motor,  which  may  be  put  out  of  sight  under  the  floor,  is  operated  by 
electric  current  from  a  storage  battery,  the  cells  of  which  are  placed 
under  the  seats  and  under  the  floor  so  as  to  act  as  ballast.  The  boat 
is  not  as  independent  as  a  steam  or  naptha  launch,  because  the  stor- 
age battery  must  be  charged  every  day  to  keep  it  in  good  order  for 
operating,  but  wherever  current  can  be  obtained  for  charging 
the  batteries,  electric  launches  are  very  convenient  and  popular. 

Copyrighted,  1895, 


255 


The  National  School  of  Electricity. 

REVIEW  OF  LESSON      XXVIII. 

Points  for  Review.     1.     For  what  purposes  may  electric  motors  be  advantageously 
used? 

2.  Why  are  electric  motors  particularly  advantageous  for  use  in  machine  shops? 

3.  What  causes  the  great  waste  of  power  in   manufactories  as  they  are  ordinarily 
arranged? 

4.  What  advantages  result  from  removing  shafts  and  belts  from  a  shop? 

5.  What  is  the  ideal  arrangement  of  motors  in  a  shop? 

6.  What  is  the  commonly  adopted  arrangement? 

7.  How  may  the  power  required  to  operate  a  shop  be  estimated? 

8.  Why  does  wood-working  machinery  require  more  power  to  drive   it  than  iron- 
working  machinery? 

9.  How  much  power  will  a  belt  drive  satisfactorily  when   running  at   the  speeds 
common  in  manufactories? 

10.  Why  does  the  width  of  the  belt  attached  to  a  machine  give  uncertain  evidence 
of  the  amount  of  power  which  is  required  to  drive  the  machine? 

1 1.  Why  do  passenger  elevators  ordinarily  require  more  power  in  proportion  than 
freight  elevators? 

12.  What  are  electric  launches? 


LKSSON         XXIX 

ELECTRIC  RAILWAYS. 

The  application  of  electric  motors  which  probably  is  most  gen- 
erally known  and  appreciated  is  in  propelling  the  electric  street  cars 
which  are  now  to  be  found  in  nearly  every  city  of  fair  size  in  this 
country.  When  the  first  great  electric  railway  enterprise  was  under- 
taken in  the  year  1888  in  Richmond,  Va.,  prophecies  of  failure  were 
numerous  and  the  discouragements  met  by  the  promoters  of  the 
enterprise  were  at  times  sufficient  to  dishearten  almost  any  one. 
Before  the  equipment  of  that  electric  railway  was  undertaken,  various 
experimental  electric  railways  had  been  laid  and  operated,  and  several 
had  been  actually  constructed  for  the  regular  carrying  of  passengers, 
but  none  of  them  were  of  such  magnitude  as^  the  railway  at  Rich- 
mond and  none  served  to  prove  the  adaptability  of  electric  motors  to 
the  purpose  of  driving  cars  as  did  the  equipment  which  was  operated 
there. 

The  first  electric  railway  which  was  really  on  a  commercial  scale 
was  a  small  line  built  in  Berlin,  Germany,  in  1879  by  the  great  firm 
of  Siemens  and  Halske.  In  1883,  the  first  electric  railway  opened 
to  the  public  in  the  United  States,  was  operated  in  the  gallery  of  the 
Chicago  Railway  Exposition  on  a  track  about -1,500  feet  long  and  of 
three  feet  gauge.  This  electric  line  caused  a  great  stir  in  the 
country  and  carried  many  passengers  who  visited  the  Exposition. 
The  motor  car  which  ran  on  the  line  weighed  three  tons  and  was 


capable  of  running  at  a  speed  of  nine  miles  an  hour.  It  was  therefore 
quite  small  compared  even  with  the  smallest  of  electric  street  cars  of 
today,  which  often  weigh  eight  or  ten  tons  and  run  at  a  speed  of 
eighteen  or  twenty  miles  an  hour.  Even  the  striking  though  modest 
attempts  at  electric  railroading  made  in  Berlin  and  Chicago  did  little 
to  bring  electric  cars  into  general  use  though  they  did  serve  to  stir  up 
the  interest  of  the  people.  The  construction  of  the  early  machines, 
as  viewed  today,  was  unmechanical  and  inefficient  so  that  great 
improvements  were  required  before  the  electric  cars  could  replace 
horse  cars  or  cable  cars.  Since  1883  the  electric  car  has  passed 
through  a  period  of  marked  development  both  in  this  country 
and  Europe.  From  the  beginning  of  1883  until  1888  many  small 
electric  railways  were  put  into  operation  in  this  country  under  the 
direction  of  Daft,  Van  Depoele,  Sprague  and  others,  but  until  the 
latter  date,  the  electric  car  cannot  be  said  to  have  proved  itself  a  com- 
mercial success.  From  1888  to  the  present  day,  electric  stret-t  rail- 
ways have  grown  in  number  and  in  favor  with  remarkable  rapidity. 
So  much  is  this  true  that  the  street  car  horse  has  been  banished  from 
the  streets  of  many  cities,  and  electric  cars  have  replaced  cable  cars 
even  in  such  cities  as  Omaha,  Neb. ,  Kansas  City,  Mo. ,  Grand  Rapids, 
Mich.,  Baltimore,  Md. ,  San  Francisco,  Cal.,  and  elsewhere. 

The  principle  of  the  electric  railway  is  very  well  illustrated  by 
Fig.  293.  In  this  figure,  A  is  a  dynamo,  one  pole  of  which  is  con- 
nected through  a  switch  and  fuse  blocks  to  the  street  railway  track, 
and  the  other  pole  to  a  wire  called  the  trolley  wire  which  is  supported 
over  the  track.  The  motor  which  drives  the  car  is  placed  under- 
neath the  floor,  as  is  shown  at  M  in  the  figure,  and  is  so  geared  to 
the  axles  that  by  the  revolution  of  its  armature  the  car  is  moved 
along.  In  order  that  current  may  be  supplied  to  the  motor,  a  mov- 
able arm  extends  above  the  car  and  presses  a  small  wheel  against  the 
trolley  wire.  This  arm  is  called  the  trolley,  and  the  current  is  con- 
veyed along  it  and  thence  down  to  the  motor.  After  the  current  has 
passed  through  the  motor,  it  completes  its  circuit  by  returning  to  the 
dynamo  through  the  rails. 

The  motors  which  are  used  on  electric  cars  are  series  wound 
(Lesson  XX,  page  160)  and  their  speed  is  controlled  either  by  means 
of  a  resistance  which  is  placed  in  circuit  with  the  motor,  or  by  some 
equivalent  device.  The  motors  are  of  various  forms  but  those  which 
are  now  commonly  iised  are  completely  ironclad,  so  that  the  armature 
is  protected  from  mechanical  injury  or  from  being  splashed  by  water 
from  the  track  (Lesson  XX,  page  165).  Nearly  all  of  the  street 
railway  motors  that  are  now  used  are  arranged  so  that  the  top  and 
bottom  halves  of  the  ironclad  fields  may  be  easily  separated  to  enable 
repairs  to  be  made  to  the  armature  or  to  the  field  coils.  This  is  a 
very  important  point  to  the  electric  railway  owner,  because  railway 


257 


service  is  very  hard  on  electric  motors.  The  machines  are  exposed 
to  dust  and  dirt  and  are  often  forced  to  do  more  work  than  that  for 
which  they  were  designed.  On  account  of  the  cramped  space  which 
is  to  be  found  under  a  street  car,  the  motors  must  be  as  compact  and 
at  the  same  time,  as  light  as  possible.  These  conditions  combine  to 
make  repairs  frequent  and  very  expensive,  unless  the  various  parts 
are  arranged  so  that  they  may  be  easily  accessible.  In  Fig.  173  the 
top  part  of  the  motor  frame  is  shown  thrown  back  so  that  the  arma- 
ture is  exposed.  The  same  thing  is  seen  in  Figs.  294  and  295 
which  show  street  railway  motors  of  other  types. 

The  axle  bearings  of  horse-cars  are  usually  attached  directly  to 
the  framework  of  the  car  floor,  and  the  same  thing  is  done  in  cars 
that  are  intended  to  be  drawn  after  electric  motor  cars  as  trailers 
or  tow  cars.  Such  a  construction  is  not  sufficiently  substantial  in 
electric  motor  cars  and  the  axle  bearings  are  mounted  on  a  strong 
iron  framework  which  is  called  a  truck  (Fig.  296).  Upon  the  top 
frame  of  this  truck  is  set  the  car  -body,  while  the  motors  are  usually 
supported  from  the  axles  and  the  truck  framework,  as  shown  in  Fig. 
296.  It  is  common  practice  to  place  two  motors  on  each  ordinary 
motor  car,  one  being  slung  on  each  axle.  This  is  done  so  as  to  use 
as  fully  as  possible  all  the  weight  of  the  car,  in  order  to  give  the 
driving  wheels  a  grip  on  the  rails.  When  one  motor  is  used  which 
is  geared  to  but  one  axle,  the  wheels  are  likely  to  slip  in  bad  weather 
or  when  the  car  is  on  grades,  and  the  speed  of  the  car  is  retarded  or 
its  progress  may  even  be  stopped  altogether.  Some  inventors  have 
arranged  gearing  so  that  one  motor  may  drive  both  axles  (Fig.  297) 
but  such  arrangements  have  never  proved  successful  when  put  into 
the  very  hard  service  to  which  the  electric  car  is  subjected. 

In  the  operation  of  electric  railway  motors,  we  have  for  the  out- 
going electric  conductor  the  overhead  trolley  wire,  and  for  the 
returning  current  the  rails  furnish  a  path.  An  electric  railway  motor 
is  therefore  in  an  electrical  position  which  is  entirely  similar  to  that 
of  an  ordinary  motor  which  is  moved  about,  and  the  lead  wires  of 
which  are  slid  along  the  electric  mains.  Railway  motors  are  almost 
always  connected  in  parallel  across  a  constant  pressure  circuit.  The 
pressure  used  is  about  500  volts.  Electric  railways  often  reach  out 
so  far  from  the  power  station  at  which  the  electric  current  is  gener- 
ated that  a  lower  pressure  is  not  practical  on  account  of  the  great 
amount  of  copper  which  would  be  required  to  carry  the  current  with 
a  reasonable  loss  of  power.  On  the  other  hand,  a  pressure  higher 
than  500  or  600  volts  would  be  unsafe  to  use  on  circuits  which 
include  bare  wires  suspended  over  the  streets.  The  pressure  of  500  volts 
is  sufficient  to  give  a  severe  shock  but  it  is  not  dangerous  to  human 
life,  as  has  been  proved  by  long  experience,  though  horses  and  some 
other  animals  which  are  more  sensitive  to  electric  shocks  than  are 


258 


FIG.  294. 


FIG.  295. 


260 


FIG.  297. 


FIG.  299. 


261 


FIG.  298A. 


FIG.  298s 


i       Ti 

_J^_                \    \                           r~\_ 

n 

411                                 X 

j 

PIG.  300. 


\/ 


\/ 


\  / 


/\ 


/  \ 


/  \ 


FIG.  301. 


FIG.  302. 


263 


^' 

XTNIVERS 


human  beings  have  been  killed  by  shocks   from    electric   railway 
wires. 

The  trolley  wire  which  is  commonly  used  consists  of  a  conductor 
of  hard  drawn  copper  No.  o,  B.  &  S.  gauge  in  size,  which  is  sus- 
pended from  span  wires  or  brackets  supported  on  poles  (Figs.  298A 
and  2986).  When  the  distances  over  which  current  must  be  carried 
are  so  great  that  a  No.  o  wire  is  of  insufficient  conducting  capacity, 
feeders  may  be  run  from  the  power  station  to  various  feeding  points 
where  they  are  connected  to  the  trolley  wire.  The  trolley  wire  then 
serves  the  same  purpose  in  the  distribution  for  the  electric  railway 
that  mains  do  in  electric  light  distributing  systems. 

The  conducting  capacity  of  the  track  must  also  be  carefully 
looked  after  even  in  the  shortest  lines.  The  rails  of  which  the  track  is 
composed  are  about  thirty  feet  long  and  their  ends  are  mechanically 
connected  by  means  of  joint  plates  or  fish  plates  and  bolts  (Fig.  299). 
On  account  of  the  scale  which  is  found  on  the  rails  and  fish  plates, 
the  joints  do  not  conduct  electricity  satisfactorily  and  it  is  necessary  to 
join  the  rails  electrically  as  well  as  mechanically.  For  this  purpose, 
what  is, called  a  bond  is  used.  A  bond  is  a  short  piece  of  copper  wire, 
the  ends  of  which  are  riveted  into  the  adjoining  ends  of  two  rails,  and 
it  thus  serves  to  make  a  good  electrical  connection  between  them  (Fig. 
300).  Sometimes  a  copper  or  an  iron  wire  is  placed  in  the  ground 
between  the  rails  and  each  rail  is  connected  to  it  by  means  of  a  bond 
(Fig.  301),  and  the  electrical  connection  between  the  rails  is  made  by 
means  of  this  continuous  wire. 

The  electric  motor  has  also  found  a  place  in .  railway  service 
which  is  much  heavier  than  that  of  the  ordinary  surface  street  rail- 
ways. After  working  its  way  into  favor  on  street  railways,  it  came 
rapidly  into  use  upon  light  suburban  railways  and  is  now  looked 
upon  as  an  essential  feature  of  any  new  system  of  city  rapid  transit. 
Possibly  one  of  the  most  striking  examples  of  the  use  of  electric 
motors  upon  rapid  transit  systems  is  on  one  of  the  underground  rail- 
roads in  the  city  of  L,ondon,  where  electric  locomotives  are  used  to 
draw  the  trains,  to  the  great  improvement  of  the  atmosphere  and 
cleanliness  of  the  tunnels.  The  equipment  of  this  railway  was  fol- 
lowed by  the  operation  of  the  Intramural  Railway  at  the  World's 
Fair  in  1893,  and  that,  by  an  elevated  railway  in  Liverpool,  England. 
In  this  country,  there  is  now  in  operation  the  great  system  of 
the  Metropolitan  Elevated  Railroad  in  Chicago,  and  several  other 
elevated  and  city  rapid  transit  railways  are  planned  in  which  electric 
motors  are  expected  to  play  a  prominent  part.  The  list  includes  the 
great  underground  railroad  system  which  is  to  be  built  to  give  the 
inhabitants  of  New  York  city  a  satisfactory  means  of  transportation 
from  their  business  places  down  town  to  homes  located  a  number  of 
miles  away  to  the  North. 


264- 


Even  this  does  not  set  the  limit  to  the  field  of  the  electric  motor 
when  applied  to  railway  purposes.  It  has  been  arranged  that  the 
heavy  trains  of  the  Baltimore  &  Ohio  Railroad  shall  be  drawn  by 
means  of  electric  locomotives  through  the  great  tunnel  just  com- 
pleted under  the  city  of  Baltimore,  and  the  locomotives  for  the  pur- 
poses have  already  been  built.  One  is  shown  in  Fig.  302.  It  is  now 
generally  believed  that  the  electric  car  will  invade  many  parts  of  the 
field  which  has  heretofore  been  exclusively  occupied  by  the  steam 
locomotive,  and  that,  in  many  kinds  of  service,  the  electric  motor 
will  as  completely  displace  steam  locomotives  as  it  has  already  dis- 
placed horse-cars  and  cable  cars  in  the  smaller  cities.  Experiments 
have  even  been  made  with  a  view  of  placing  electric  locomotives  in 
service  upon  main  trunk  railway  lines,  and  the  superintendent  of  an 
important  English  railway,  it  is  said,  believes  he  could  quickly 
change  his  whole  system  from  one  using  steam  locomotives  to  one 
using  electric  locomotives  if  the  officers  of  the  road  so  directed.  Be 
this  as  it  may,  the  fact  is  plain  that  the  electric  motor  has  made  a 
wonderful  record  for  itself  when  used  upon  electric  railways  in  the 
past  and  that  its  record  will  be  much  more  remarkable  in  the  future. 

Copyrighted,  1895, 


265 


The  National  School  of  Electricity. 

REVIEW  OF  LESSON      XXIX. 

Points  for  Review.     1.  When  was  the  Richmond  electric  railroad  constructed? 

2.  When  and  where  was  the  first  electric  railroad  built  on  a  commercial  scale? 

3.  When  and  where  was  the  first  public  electric  railroad  built  in  the  United  States? 

4.  What  is  a  trolley  wire?     What  is  a  trolley? 

5.  What  is  a  truck?     Why  are  trucks  used  under  electric  cars? 

6.  Why  are  two  motors  usually  used  on  electric  cars? 

7.  Is  the  electric  railway  current  dangerous? 

8.  Why  is  it  necessary  to  "  bond"  the  rails  of  electric  railways? 

9.  To  what  purposes  have  heavy  electric  locomotives  been  put? 


LKSSON     XXX. 

METHODS  OF  HANDLING  AND  CONTROLLING  RAIL- 
WAY MOTORS  AND  GENERATORS. 

The  question  of  getting  the  greatest  possible  amount  of  work 
out  of  his  machinery  and  at  the  same  time  of  expending  the  smallest 
practicable  amount  of  money  for  its  safe  operation,  is  one  which  weighs 
continually  on  the  mind  of  the  manager  of  every  great  electric  plant. 
It  is  this  which  leads  him  to  watch  all  expenditures  and  keep  an 
accurate  account  of  all  the  supplies  used  in  his  station.  The  accounts 
show  him  the  cost  of  fuel,  oil.  water,  labor,  and  other  items  for  every 
1,000  watts  generated  for  an  hour  by  the  dynamos.  By  comparison 
of  these  records  month  by  month,  and  with  the  records  of  other  plants 
of  similar  size,  it  is  possible  to  tell  whether  every  possible  economy 
is  practiced  which  will  not  cause  oppression  to  the  employees  or 
injury  to  the  plant.  The  record  of  the  output  of  a  station  is 
usually  made  by  the  switchboard  attendant,  who,  every  quar- 
ter or  half  hour,  enters  the  reading  of  the  feeder  amperemeters 
and  of  the  voltmeters  in  a  large  book  which  is  properly  ruled. 
Sometimes  the  record  is  made  by  automatic  instruments.  Fig. 
323,  for  instance,  is  a  reproduction  of  the  card  taken  from  a 
recording  voltmeter  which  is  used  in  a  large  central  sta- 
tion, for  electric  lighting.  The  card  shows  the  continuous 

266 


record  of  the  pressure  which  was  maintained  at  the  centers  of  distri- 
bution during  twenty-four  hours.  The  distance  between  two  suc- 
cessive radial  lines  represents  fifteen  minutes,  and  the  distance  along 
the  radial  lines  included  between  any  two  circles  represents  two  volts. 
Recording  amperemeters  are  not  as  commonly  used  as  are  recording 
voltmeters,  as  the  voltmeter  record  is  a  check  upon  the  care  with 
which  the  pressure  is  kept  constant,  while  there  is  no  particular 
need  of  keeping  an  extremely  exact  record  of  the  current.  Fig.  324 
shows  the  current  sent  out  from  a  certain  electric  light  station  during 
twenty-four  hours.  The  hours  of  the  day  and  night  are  laid  off  on 
the  horizontal  line  and  the  current  at  any  hour  is  equal  to  the  length 
of  the  corresponding  vertical  line  which  is  included  between  the 
horizontal  line  and  the  irregular  line.  This  shows  very  plainly  the 
effect  of  the  dark  hours  of  the  afternoon,  in  causing  a  great  in- 
crease in  the  demand  for  light. 

The  total  amount  of  current  which  is  required  by  the  customers 
of  an  electric  light  plant  changes  from  hour  to  hour  with  compara- 
tive slowness,  as  is  shown  by  Fig.  324,  and  such  an  amount  of 
machinery  can  be  kept  running  at  all  times  as  will  supply  the  load 
most  economically.  A  very  different  condition  exists  in  the  power 
house  which  supplies  current  to  electric  street  cars.  Fig  325  shows 
the  amount  of  current  sent  out  during  one  hour  from  an  electric 
railway  power  house,  the  record  being  laid  out  in  the  same  way  as 
that  of  Fig.  324.  This  figure  shows  the  wonderful  range  and 
rapidity  of  the  changes  in  the  current  supplied  by  the  station.  Since 
compound  wound  dynamos  which  keep  the  pressure  fairly  constant 
are  used  in  such  stations,  the  variations  of  the  current  cause  similar 
variations  of  the  load  on  the  dynamos  and  engines.  Every  effort  has 
been  made  to  reduce  the  range  of  these  changes  which  cause  shocks 
to  the  machinery  and  so  are  likely  to  finally  result  in  injury  or  break- 
down, and  which  also  make  it  impossible  to  keep  the  machinery 
sufficiently  well  loaded,  so  that  it  may  be  operated  with  the  greatest 
economy.  One  method  which  has  been  put  on  trial  with  a  view  to 
decreasing  the  great  changes  in  the  load  on  railway  stations  calls  tor 
the  use  of  a  storage  battery.  This  battery  has  its  positive  terminal 
connected  directly  to  the  positive  'bus  bar  and  its  negative  terminal 
to  the  negative  'bus  bar;  then,  when  a  great  demand  for  current  is 
made  by  the  cars,  part  of  it  is  supplied  by  the  battery,  and  the 
dynamos  and  engines  are  thus  relieved  to  some  extent.  When  the 
current  required  by  the  cars  is  small,  the  battery  takes  current  from 
the  dynamos,  by  which  means  it  is  kept  charged,  and  thus  the  varia- 
tions of  the  load  are  made  much  smaller  than  they  would  be  without 
the  battery.  This  plan  has  not  proved  very  successful  because  the 
storage  battery  is  too  frail  to  stand  hard  service,  but  when  a  satis- 
factory battery  is  developed  it  will  fill  an  excellent  place.  Batteries 


267 


are  also  used  in  one  or  two  large  American  and  several  foreign 
electric  light  stations  to  aid  in  supplying  the  current  during  the 
period  of  greatest  load,  and  the  batteries  are  then  re-charged  during 
the  period  of  light  load.  Batteries  are  more  likely  to  last  a  reason- 
able length  of  time  when  used  in  such  service,  but  even  here  they 
have  not  been  sufficiently  successful  to  come  into  much  use. 

The  improvements  which  have  the  greatest  effect  upon  the  loads 
of  electric  railway  power  stations  are  in  the  street-car  motors  and 
especially  in  the  way  in  which  they  are  controlled.  The  earlier 
motors  which  were  put  upon  street  cars  were  wired  up  so  that  the 
two  machines  were  put  permanently  in  parallel,  and  they  were  then 
controlled  by  means  of  resistances  put  in  series  with  them.  A  great 
many  cars  are  still  con  trolled,  in  this  manner.  When  the  car  is  to  be 
started,  a  controller  lever  is  moved  so  that  it  connects  the  two  motors 
to  the  circuit  in  series  with  a  resistance.  To  make  the  cars  run 
faster,  the  resistance  is  gradually  cut  out  of  the  circuit,  and  finally  a 
certain  portion  of  the  series  field  coils  of  the  motors  may  be  cut  out 
also,  if  a  particularly  high  speed  is  desired.  The  commonest  form 
of  rheostat  is  that  known  as  the  Thomson-Houston  street  car  con- 
troller, which  is  shown  in  Fig.  326.  It  is  shown  connected  to  the 
motors  in  Fig.  327. 

Another  way  of  controlling  the  speed  of  street  cars  is  by  what  is 
called  the  "commutated  field"  method.  In  this  case,  the  fields  of 
the  motors  are  wound  in  separate  divisions,  usually  three  in  number, 
and  the  speed  of  the  motor  is  controlled  by  connecting  the  field  coils 
of  each  motor  in  different  combinations.  This  is  indicated  in  the 
diagram  of  Fig.  328,  where  +A  and  — A  represent  the  armature 
terminals  of  a  motor,  and  +c,  — c,  -fa,  — a,  +b,  — b  represent 
the  terminals  of  the  field  divisions.  The  connections  of  the  field  are 
changed  or  u  commutated  "  by  means  of  a  controller  or  switch  which 
consists  of  a  wooden  cylinder  or  barrel  on  which  are  placed  brass 
plates  of  various  shapes.  This  is  shown  developed  (rolled  out  flat)  in 
Fig.  329,  and  the  forms  of  the  plates  are  well  shown.  These  plates 
bear  against  spring  contact  buttons  set  in  a  row  at  the  back  of  the 
switch  box,  each  one  of  which  is  connected  by  a  wire  to  one  of  the 
terminals  at  the  motor.  Fig.  330  shows  the  buttons  with  the  wire 
connections  which  run  in  a  cable  from  the  switch  on  the  car  platform 
to  the  motors  under  the  car.  When  a  car  is  to  be  started,  the  switch 
handle  is  moved  and  the  motors  are  connected  with  their  individ- 
ual field  coils  in  series  as  indicated  in  Fig.  331.  To  run  the  car 
faster,  the  lever  is  moved  from  point  to  point,  commutating  the 
fields  into  various  arrangements,  until  on  the  seventh  and  last  notch 
the  individual  field  coils  are  in  parallel.  The  various  arrangements 
of  the  field  coils  when  the  switch  stands  at  the  various  points  are 
shown  in  Fig.  332. 


368 


u 

C,  C,  Switches  in  motor  circuit.  D,  Switch  in  lamp  circuit.  E,  reversing  switch. 
P,  fuse  blocks  in  lamp  circuit.  G,  fuse  block  in  motor  circuit. .  H,  H,  H,  incandes- 
cent lamps.  K,  K,  controllers.  O,  lightning  arresters.  M,  rheostat.  N,  trolley. 

IG.  327. 


OF  THE 


3  v 


FIG.  330. 


l\v\\\ 


FIG.  329. 


Trolly 


Field 


Armaturfe 


Ground. 
FIG.  331. 


FIG.  332. 


I    /    \ 


SHOWING 
VARIATION  OF  CURRENT 

WITM 

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1 1 1~ 


CONTROL. 

WEIBHT   Or    CAR.  TOTAL  1*000 1 bi          J 
,ui».d     wit^   TWO   B-C.,aOD  M.t«^    1 


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3     4    5     6     7     8     8      10   II     12    13    14    15    16.  17    18    19   £0  21   22   232429 

SECONDS 

Fie.  334. 


373 


FIG.  312. 


FIG.  313. 

274. 


FIG.  333. 


275 


SE    LIBRA f 
UNIVERSITY 


Both  of  the  earlier  forms  of  controllers  serve  very  well  as  far  as 
handling  the  cars  is  concerned,  but  the  use  of  resistances  causes  a 
great  waste  of  power,  and  consequently  the  cars  require  a  great 
deal  of  current  in  starting.  This  in  turn  has  an  effect  in  increasing 
the  suddenness  and  magnitude  of  the  changes  of  load  at  the  power 
station.  The  need  for  a  more  efficient  controller  which  would  waste 
less  power  and  allow  the  cars  to  start  with  less  current  became  so 
pressing  that  various  devices  were  designed  to  meet  the  want.  All  of 
these  were  reduced  to  some  form  of  "  series  parallel  "  controller  which 
is  now  used  on  a  great  majority  of  electric  cars.  With  this  form  of 
controller,  the  motors  are  connected  in  series  with  each  other  when  the 
car  is  started,  and  are  then, connected  in  parallel  when  it  is  desired  to 
run  the  car  rapidly.  From  this  use  of  the  motors  both  in  series  and  in 
parallel,  came  the  name  "series-parallel"  controller.  The  pull  or 
torque  with  which  a  series  wound  motor  tends  to  start,  depends  only 
upon  the  current  flowing  through  it.  If  two  motors  be  connected  in 
parallel  and  enough  current  be  passed  through  them  to  start  a  street- 
car, the  tot?l  amount  of  current  may  be  as  much  as  80  amperes.  The 
starting  effort  in  this  case  is  caused  by  forty  amperes  flowing  through 
each  motor.  Now,  if  the  same  two  motors  be  connected  in  series  with 
each  other  and  a  current  of  forty  amperes  is  permitted  to  flow  through 
them,  each  will  exert  the  same  starting  effort  as  before,  and  the  car  will 
start  with  the  expenditure  of  only  half  the  current.  Having  started  the 
car,  the  motors  must  be  connected  in  parallel  in  order  that  it  may  run 
at  a  reasonably  high  speed,  because  when  the  motors  are  in  series  the 
total  pressure  of  500  volts  is  divided  between  them  and  each  there- 
fore gets  only  about  250  volts.  The  speed  of  a  motor  depends 
directly  upon  the  pressure  at  its  armature  terminals  and  therefore 
when  connected  in  series,  the  motors  will  run  at  only  half  speed. 

The  actual  process  of  controlling  a  car  by  the  series  parallel 
method  consists  of  starting  the  car  with  a  resistance  and  the  motors 
in  series,  cutting  the  resistance  out  of  circuit,  and  then  by  a  series  of 
commutations  indicated  in  Fig.  333  putting  the  motors  in  parallel 
with  each  other  and  in  series  with  the  resistance.  This  resistance  is 
finally  cut  out  of  the  circuit,  and  sometimes  a  portion  of  the  field 
windings  are  cut  out  of  the  circuit  to  make  the  car  run  at  a  high 
speed. 

The  comparative  efficiency  of  operating  cars  with  motors 
equipped  with  rheostat  and  with  series-parallel  controllers  is  illus- 
trated in  Fig.  334.  The  time  after  current  is  admitted  to  the 
motors  is  laid  off  on  the  horizontal  line,  and  the  distance  from  the 
horizontal  to  the  wavy  lines  at  any  point  shows  the  amount  of 
current  flowing  through  the  motors  at  that  instant.  The  upper 
wavy  line  shows  the  current  consumed  when  a  certain  pair  of  motors 
were  controlled  by  a  rheostat,  and  the  lower  wavy  line  shows  the 


current  consumed  when  the  same  motors  were  controlled  in  the  series- 
parallel  fashion.  During  the  first  ten  seconds,  the  rheostat  control 
required  twice  as  much  current  on  the  average  as  did  the  series- 
parallel  control,  and  during  the  first  eighteen  seconds  the  rheostat 
required  one  half  more  current. 

A  similar  figure  might  also  be  drawn  to  illustrate  the  difference 
between  the  amounts  of  current  used  by  a  careful  motor  man  in 
starting  his  car  and  by  a  careless  man.  The  former  always  moves  his 
controller  lever  from  point  to  point  with  care,  and  permits  the 
motors  to  gather  speed  before  passing  from  one  point  to  a  higher  one. 
By  neglecting  this  precaution,  a  considerably  larger  current  may  be 
used  than  is  necessary.  Some  steam  railroads  pay  a  bonus  to  the 
engineer  who  succeeds  in  making  his  runs  each  month  with  the 
least  coal,  and  it  would  be  a  paying  investment  for  many  electric 
railroads  to  pay  a  bonus  to  their  motor  men  who  succeed  in  making 
the  runs  with  the  least  current. 

The  handling  of  railway  station  generators  is  all  carried  out  by 
proper  switches  and  controlling  arrangement  which  are  placed  on 
a  switchboard  to  which  connections  are  run  from  the  dynamos,  and 
from  which  the  overhead  or  underground  feeders  run. 

In  Figs.  312  and  313  are  shown  the  dynamo  and  engine  rooms 
of  a  large  railroad  power  station.  The  switchboard  for  this  station 
is  shown  in  the  distance  in  Fig.  312,  and  on  the  left  in  Fig.  313.  A 
diagram  of  the  connections  on  this  switchboard  is  shown  in  Fig. 
314.  The  total  length  of  the  switchboard  is  96  feet,  and  it  is  of  suf- 
ficient size  to  carry  the  connections  and  controlling  devices  for  two 
hundred  and  fifty  feeders  and  the  generators  which  furnish  current 
to  the  feeders. 

On  the  lower  panels  of  the  switchboard  are  the  amperemeters, 
automatic  circuit  breakers,  switches,  voltmeters  and  rheostats  for 
the  dynamos,  while  on  the  upper  panels  are  the  feeder  switches,  am- 
peremeters, circuit  breakers  and  lightning  arresters.  Below  the 
platform,  between  the  two  panels,  may  be  seen  the  positive  and  neg- 
ative bus  bars  and  the  equalizer  bar.  The  point  of  contact  between 
the  compound  coil  of  the  series  field  and  the  dynamo  brush  in  each 
machine  is  connected  with  this  equalizer  bar  in  order  to  prevent  any 
of  the  compound  wound  dynamos  taking  more  than  their  share  of 
the  load.  The  three  (positive,  negative  and  equalizer)  terminals 
from  each  generator  run  to  a  3-pole  switch  on  one  of  the  lower 
panels  of  the  switchboard,  so  that  when  the  generator  is  discon- 
nected from  the  bus  bars  all  three  of  the  wires  may  be  opened  at  the 
same  time  by  one  switch. 

The  main  positive  bus  bars  which  are  shown  just  below  the 
platform  are  connected  through  eight  4,000  ampere  switches  and 
amperemeters  to  the  feeder  bus  bars  which  are  shown  back  of  the 
upper  part  of  the  upper  panels.  From  these  feeder  bus  bars,  taps 


are  made  through  switches,  amperemeters  and  circuit  breakers  to 
the  individual  feeders.  The  bus  bars  are  nearly  all  of  them  3-inch 
round  copper,  21  feet  long,  supported  on  suitable  insulation,  and 
taps  are  connected  to  them  by  means  of  split  collars  which  are 
clamped  on. 

The  present  boiler  capacity  of  this  plant  is  3,750  horse  power, 
and  the  engines  and  generators  have  a  capacity  of  6,000  horse  power. 
Two  hundred  and  seventy  cars  are  now  operated  from  this  station. 
About  60  tons  of  anthracite  pea  coal  are  consumed  every  day,  and 
three  firemen  and  two  cleaners  are  kept  at  work  continuously.  As 
will  be  seen  in  Figs.  312  and  313,  the  armatures  of  the  dynamos  are 
each  built  up  on  the  main  shaft  of  a  double  Corliss  engine,  which  is 
the  most  economical  arrangement  for  driving  large  dynamos  which 
is  possible.  The  dynamos  run  at  a  speed  of  about  80  revolutions  a 
minute,  and  each  one  has  10  pole  pieces  placed  radially  on  its  circu- 
lar yoke  which  has  an  outside  diameter  of  12  feet  6  inches.  Bach 
armature  is  90  inches  and  its  commutator  is  60  inches  in  diameter. 

Copyrighted,  1895, 


The  National  School  of  Electricity. 

REVIEW    OF    LESSON    XXX. 

Points  for  Review.     1.     How  are  output  records  kept  in  large  electric  stations? 

2.  Why  are  recording  voltmeters  particularly  useful? 

3.  In  what  special  respect  does  the  load  of  an  electric  railway  power  plant  differ 
from  that  of  an  electric  light  plant? 

4.  How  may  a  storage  battery  be  used   to  smooth  the  ' '  load  curve  "  of  an  electric 
station? 

5.  What  methods  are  used  for  controlling  street-car  motors? 

6.  Why  is  the  series-parallel  method  more  economical  than  the  rheostat  or  corn- 
mutated  fields  method? 

7.  What  is  the  essential  point  in  the  series-parallel  method  of  control  for  street-car 
motors? 

8.  In  what  way  can  a  motor-man  economize  in  the  current  used  by  his  car? 

9.  What  instruments  are  placed  on  a  station  switchboard? 

10.     What  is  the  object  of  the  third  bus  bar,  or  "equalizer,"  when  compound  dyna- 
mos are  connected  in  parallel? 


XXXI. 

MODEL  ELECTRIC  PLANTS. 

When  Mr.  J.  E.  H.  Gordon  wrote  a  Practical  Treatise  on  Electric 
Lighting  in  1884,  he  filled  the  rather  large  book  with  descriptions 
of  dynamos  and  electric  lamps  made  in  forms  which  are  now  nearly 
all  discarded,  but  at  that  time  there  was  little  else  to  write  about  in 
respect  to  the  question  of  electric  lighting.  There  were  at  that  time 
no  great  electric  lighting  plants  such  as  we  have  today,  nor  were 
there  any  even  to  be  compared  with  those  in  existence  only  five 
years  later  than  the  date  of  the  book.  With  the  same  courage  and 
optimism  which  led  him  to  say  in  1881,  "the  day  will  come  when 
gas-light  will  be  as  obsolete  as  wooden  torches,  and  when  in  every 
house  the  incandescent  lamp  will  have  replaced  the  gas  jet,"  Mr. 
Gordon  left  space  in  his  book  for  a  chapter  called  Central  Station 
Lighting.  Under  the  heading  was  only  the  single  paragraph,  UI 
had  intended  to  write  a  long  chapter  with  the  above  heading,  but, 
for  various  rersons,  I  am  not  yet  prepared  to  do  so.  I  have,  how- 
ever, left  in  the  heading  for  the  convenience  of  inserting  such  a 
chapter  in  a  future  edition  of  this  book,  should  one  ever  be  required." 

At  the  present  time,  or  ten  years  later  than  the  time  when  Mr. 
Gordon  wrote,  we  have  numbers  of  books  upon  the  subjects  of  elec- 
tric lighting  and  electric  plants,  and  the  progress  of  the  decade  has 
been  so  enormous  that  many  of  the  descriptions  in  Gordon's  books 
seem  to  belong  to  another  age.  We  may  say,  indeed,  that  they  do 
belong  to  another  age,  for  ten  years  constitutes  an  epoch  in  the 
history  of  the  modern  development  of  electricity. 

279 


It  is  instructive  and  interesting  to  see  the  way  in  which  electric 
plants  have  developed  since  1884.  The  development  is  best  shown 
by  figures  representing  plants  which  were  built  at  different  periods. 
Fig.  303  shows  one  of  the  earliest  electric  light  plants  of  the  world, 
the  first  Edison  central  station  for  the  public  supply  of  electric  cur- 
rent, which  was  located  at  Appleton,  Wis.,  in  1881.  At  the  left 
hand  of  the  figure  is  shown  the  exterior  of  a  small  frame  shanty  in 
which  this  plant  was  located,  while  at  the  right  hand  of  the  figure 
the  shanty  is  shown  with  one  side  removed  so  that  the  plant  with  its 
dynamo,  pulleys  and  belts  is  exposed  to  view.  This  plant  was  operated 
by  water  power  and  the  gears  on  the  water  wheel  shaft  used  to  drive 
the  counter  shafts  to  which  the  dynamo  was  belted,  are  shown  in  the 
center  of  the  figure.  This  plant  was  put  in  operation  before  the  day 
of  the  three-wire  system,  and  it  therefore  has  only  one  dynamo. 
Behind  the  dynamo  in  the  figure,  the  regulating  and  indicating 
apparatus  are  vaguely  seen.  A  peculiar  and  interesting  point  in  the 
figure  is  the  dynamo  which,  it  will  be  noticed,  looks  quite  different 
from  those  illustrated  in  preceding  lessons.  This  dynamo  has  a 
spindling,  lean  appearance  which  forms  a  decided  contrast  to  the 
chunky,  substantial  appearance  of  the  modern  dynamos.  The  field 
magnets  of  the  dynamo,  which  is  bipolar,  are  divided  into  several 
legs  as  though  there  were  several  separate  horse-shoe  electromagnets 
attached  to  the  poles.  At  the  time  these  machines  were  built,  this 
was  supposed  to  be  the  best  way  of  constructing  dynamos,  but  the 
modern  construction  with  a  single  short  horse-shoe  has  been  proved 
to  be  the  best  form  for  bipolar  dynamos  with  salient  poles.  One  of 
these  old  so-called  u spindle  shank"  dynamos  which  was  used  by 
Mr.  Edison  in  his  first  public  exhibition  of  incandescent  electric 
lights  at  Menlo  Park  in  1880,  is  now  in  the  dynamo  collection  of  the 
University  of  Wisconsin,  where  it  makes  a  striking  contrast  to  the 
appearance  of  the  substantial  later  dynamos  of  equal  capacity  which 
stand  by  its  side.  Notwithstanding  its  peculiar  appearance,  the  old 
dynamo  is  still  good  for  any  reasonable  service,  and,  indeed,  it  had 
been  doing  almost  daily  work  from  1880  up  to  the  time  of  the 
World's  Fair,  where  it  was  exhibited,  and  from  whence  it  was  for- 
warded to  its  present  place.  One  of  the  old  spindle  shanks  with  a 
modern  dynamo  of  the  same  capacity  beside,  it,  is  shown  in  Fig.  304. 

The  plant  which  is  now  located  at  Appleton,  Wis. ,  is  as  great 
a  contrast  to  the  original  one  as  the  old  dynamo  is  to  modern 
machines.  It  now  contains  several  fine  dynamos  with  excellent 
regulating  devices,  housed  in  a  substantial  building,  which  are  used 
to  furnish  current  to  incandescent  and  arc  lights,  stationary  electric 
motors,  and  to  electric  cars. 

The  great  landmark  in  electric  central  stations,  the  Pearl  Street 
station  of  New  York  city,  was  operated  continuously  from  the  fall  oi 


380 


FIG.  303. 


FIG.  304. 


OT 


FIG.  305. 


FIG.  306. 


(U: 


FIG.  307. 


283 


FIG.  308. 


FIG   309. 


284, 


FIG.  310. 


FIG.  311. 


285 


t 

™  OF  THE 

UNIVERSITY^ 


FIG.  315. 


FIG.  316. 

286 


1 882  until  a  short  time  ago,  when  it  was  destroyed  by  fire.  It  has 
now  been  replaced  by  a  magnificent  station  to  which  reference  will 
be  made  later.  Fig.  305  shows  one  of  the  great  "Jumbo"  dynamos 
which  were  used  in  this  station,  each  directly  coiipled  to  its  own 
engine.  Each  one  of  these  dynamos  had  a  capacity  of  i,5OOsixteen- 
candle  power  incandescent  lamps  and  occupied  not  less  than  175  square 
feet  of  floor  space.  It  is  interesting  to  compare  the  Jumbo  machine  with 
one  of  the  latest  triumphs  of  electrical  engineering,  the  great  "steam 
dynamo"  shown  in  Fig  306,  which  has  a  capacity  of  3,600  sixteen- 
candle  power  incandescent  lamps  and  occupies  but  little  more  floor 
space  than  the  Jumbo.  The  Jumbo  dynamos  were  wonderful 
machines  in  their  day  and  a  few  are  still  running  in  European  elec- 
tric light  stations,  but  most  of  them  were  soon  superseded  by  faster 
running  central  station  dynamos  driven  by  belts  instead  of  being 
directly  coupled  to  engines. 

This  move  in  the  line  of  construction  changed  the  arrangements 
of  city  central  stations  so  that  several  great  plants  built  in  New  York, 
Chicago,  Philadelphia  and  Boston  were  constructed  after  the  general 
plan  shown  in  Fig.  307.  This  figure  shows  a  cross  section  of  one  of 
the  central  stations  of  the  Edison  Electric  Illuminating  Company  of 
New  York  city.  Here  we  see  high  speed  steam  engines  located  in 
the  basement  so  that  they  may  be  on  a  solid  foundation,  and  from 
their  fly  wheels  belts  run  to  dynamos  located  upon  the  floor  above. 
The  two  floors  above  the  dynamos  are  occupied  by  boilers  which 
furnish  steam  to  the  engines  located  in  the  basement,  and  by  arrange- 
ments for  handling  the  ashes  which  come  from  the  boiler  furnaces. 
Above  the  boilers  is  a  floor  wholly  given  over  to  bins  for  holding  coal 
for  the  boilers,  which  is  hoisted  from  the  street  by  an  elevator.  The 
top  floor  is  given  to  repair  shops,  store  rooms,  etc.  Fig.  308  shows 
the  front  of  the  Central  Station  building.  This  Central  Station 
fairly  represents  the  type  which  has  been  used  for  a  number  of  years 
in  great  cities  where,  on  account  of  the  expense  of  land,  it  is  desira- 
ble to  occupy  as  little  ground  space  as  possible.  In  the  great  stations 
which  have  been  built  in  Chicago,  Boston  and  New  York  within 
three  or  four  years,  the  arrangement  is  made  still  more  economical. 
This  will  be  referred  to  later. 

In  the  smaller  cities  and  towns  where  land  is  not  so  valuable,  it 
is  usual  to  place  the  boilers  on  the  ground  floor  with  the  engines,  and 
the  dynamos  are  then  placed  either  upon  the  same  floor  or  on  the 
floor  above.  One  arrangement  of  a  central  station,  with  the  boilers, 
engines  and  dynamos  all  on  the  same  floor,  is  well  shown  in  Fig.  309. 
Two  engines  are  shown  in  this  with  a  dynamo  driven  by  a'  belt  from 
each  fly  wheel,  and  between  the  engines  a  shaft  is  coupled  so  that 
additional  dynamos  may  be  belted  from  its  pulleys.  A  station  with 
boilers  and  engines  on  one  floor  and  the  dynamos  on  the  floor  above 


287 


is  very  well  shown  in  Fig.  310  ^hich  is  a  cross  section  of  a  large 
plant.  Fig.  311  shows  the  boiler  room  of  another  great  plant  simi- 
larly arranged.  These  figures  are  taken  from  actual  plants  which 
are  in  successful  operation  and  their  countei  parts  may  be  seen  in  a 
great  many  cities  and  towns  in  this  country.  Each  plant  illustrated 
is  a  model  of  its  kind  and  from  that  stand  point  will  bear  the 
closest  comparison  which  the  classes  may  make  between  it  and 
plants  which  they  may  have  the  opportunity  of  examining. 

After  several  years,  during  which  small  dynamos  were  used  in 
electric  plants  belted  to  counter  shafts  or  directly  to  the  fly  wheels  of 
engines,  the  manufacturers  of  dynamos  began  again  to  make  dyna- 
mos, which,  like  the  "Jumbo ?'  machines,  should  be  directly  connected 
to  engines,  and  the  largest  central  stations  are  now  built  with  such 
machines  (Fig.  306).  The  greatest  machines  of  the  kind  ever  built, 
and  indeed  the  largest  dynamos  of  any  kind,  are  the  great  dynamos 
which  are  now  being  erected  in  the  power  house  of  the  Niagara  Falls 
Power  Company  at  Niagara  Falls. 

The  works  of  this  company  constitute  the  greatest  industrial 
power  plant  ever  constructed.  A  general  view  of  the  plant  is 
shown  in  Fig.  317.  Taking  water  from,  the  Niagara  river  above  the 
falls,  a  canal  built  for  the  power  company  by  the  Cataract  Con- 
struction Company  conducts  the  water  about  1,500  feet,  to  where  the 
water  wheels  are  located.  These  wheels  are  located  at  the  bottom  of 
an  enormous  wheel  pit  179  feet  deep,  21  feet  wide,  and  of  sufficient 
length  to  permit  the  location  of  many  very  powerful  turbine  water 
wheels.  The  water  is  conveyed  from  the  canal  on  the  surface  of 
the  ground  down  to  the  wheels  at  the  bottom  of  the  pit,  through 
great  steel  tubes  or  "  penstocks  "  seven  and  a  half  feet  in  diameter. 
After  the  water  has  passed  through  the  wheels,  delivering  up  to  them 
its  power,  it  is  carried  away  through  a  tunnel  a  mile  and  a  quarter 
long,  to  be  discharged  into  the  river  below  the  falls.  The  canals 
and  tunnels  of  the  Niagara  Falls  Power  Company  have  been  con- 
structed on  such  a  scale  that  the  amount  of  water  which  will  pass 
through  them  is  capable  of  delivering  125,000  horse-power  to  the 
water  wheels,  and  the  charter  of  the  company  permits  it  to  take  as 
much  water  as  will  give  200,000  horse-power.  The  amount  of  power 
represented  by  this  is  as  much  as  one-tenth  of  the  power  which  can 
be  developed  by  all  the  water  wheels  in  the  United  States,  and  is 
greater  than  the  water  power  of  the  following  great  power  and 
manufacturing  centres,  all  added  together:  Lawrence,  Lowell  and 
Holyoke,  Mass.;  Manchester,  N.  H.;  Lewiston,  Me.;  Bellows  Falls, 
Vt;  Rochester,  Cohoes,  Oswego  and  Lockport,  N.  Y. ;  Paterson,  N. 
J. ;  Augusta,  Ga. ;  and  Minneapolis.  Even  this  enormous  amount  of 
power  which  the  Niagara  Falls  Power  Company  proposes  to  supply 
to  its  customers  is  very  small  compared  with  the  power  which  is 
contained  by  all  the  water  in  the  falls.  If  all  the  power  represented 


388 


by  the  water  as  it  flows  from  the  upper  rapids  over  the  falls  BO  the 
lower  rapids  were  utilized,  it  would  make  about  eight  and  a  quarter 
million  horse  power,  or  more  than  four  times  as  much  as  the 
of  all  the  water  wheels  in  the  United  States,  and  considerably  moi 
than  the  combined  power  of  all  the  steam  engines  and  water  wheelsN 
which  are  used  in  this  country.  The  Niagara  Falls  Power  Company 
were  not  able  to  take  advantage  of  the  total  height  down  which  the 
water  flows,  but  if  the  power  of  all  the  water  in  the  falls  were  as 
fully  utilized  as  the  power  company  propose  to  utilize  that  of  {he 
water  which  they  pass  through  their  wheels,  it  would  still  yield  four 
million  horse-power,  or  much  more  than  half  of  all  the  power 
now  used  in  the  country.  It  is  seen  from  this  that  the  great 
plans  of  the  Niagara  Falls  Power  Company,  when  fully  carried 
out,  will  divert  only  about  one- twentieth  of  all  the  water  from 
the  falls,  and  plenty  will  remain  for  the  purposes  of  other  power 
companies,  if  the  organization  of  others  becomes  desirable,  and  yet 
leave  sufficient  water  so  that  the  grandeur  and  beauty  of  the  falls 
shall  not  be  injured. 

In  Figs.  318  and  319  are  shown  two  views  of  the  wheel  pit 
and  power  house  of  the  Niagara  Falls  Company.  The  first  figure 
shows  a  vertical  section  taken  crosswise  through  the  wheel  pit 
and  house  and  the  second  shows  a  vertical  section  taken 
lengthwise  through  the  pit  and  shows  the  positions  of  two  of  the 
•water  wheels  and  dynamos  which  are  now  being  erected.  In  the 
lower  left  hand  corner  of  the  latter  figure  is  seen  the  tail  race  tunnel 
by  which  the  water  is  discharged  into  the  river.  In  the  figures,  W 
W  are  the  water  wheels,  which  are  twin  wheels  having  the  enormous 
capacity  of  5,000  horse-power,  and  P  P  are  the  penstocks.  S  S  are 
great  hollow  steel  shafts  no  less  than  thirty-eight  inches  in  diameter, 
except  at  the  bearings  where  they  are  solid  and  eleven  inches  in  dia- 
meter. Each  shaft  conveys  the  5,000  thousand  horse-power  developed 
by  the  wheel,  to  which  it  is  attached,  to  a  great  dynamo  fastened  to 
its  upper  end.  At  C  in  the  figure,  the  canal  which  brings  water  to 
the  penstocks  is  shown,  and  at  T  is  shown  the  electric  traveling 
crane,  capable  of  lifting  fifty  tons,  which  is  placed  in  the  power 
house  to  be  used  in  placing  the  machinery  in  position  and  in  case  the 
machinery  must  be  taken  to  pieces  at  any  time  for  the  purpose  of 
repairs.  Three  of  these  ( '  generating  units  ' '  will  soon  be  ready  to 
deliver  power  to  such  mills  as  are  located  within  a  short  distance  of 
the  great  power-house.  The  5,000  horse-power  water  wheels  which 
are  over  five  feet  in  diameter  and  revolve  at  a  speed  of  250  revolu- 
tions per  minute  are  marvels  of  engineering  and  constructive  skill, 
but  we  cannot  stop  to  consider  their  details  or  the  remarkable  bear- 
ings upon  which  are  supported  the  enormous  weights  of  the  dynamo 
and  shaft  which  are  connected  to  each  wheel  and  which  amount  to 
a  total  of  some  80  tons.  The  revolving  parts  of  each  dynamo 


289 


alone  weigh  40  tons  and  are  of  the  most  massive  character.  These 
dynamos,  which  were  designed  and  built  by  the  Westinghouse  Elec- 
tric Company,  generate  a  two-phase  alternating  current  at  2,000  volts 
pressure,  having  the  quite  low  frequency  of  25  periods  per  second, 
and  it  is  expected  to  use  the  currents  for  operating  either  motors  or 
lights.  As  the  plant  is  primarily  designed  for  the  transmission  of 
power  to  factories  and  mills,  it  is  expected  that  the  greater  part  of 
the  current  will  be  used  in  operating  motors.  Thus  far,  only  three 
generating  units  of  5,000  horse-power  each  have  been  ordered  for 
the  electric  power-house,  although  some  additional  water  power  is 
now  being  furnished  directly  to  paper  mills.  The  present  unfinished 
condition  of  the  electric  power-house  is  plainly  shown  by  Fig.  320, 
which  is  from  a  photograph  lately  taken  at  Niagara.  The  figure 
shows  plainly  where  the  great  dynamos  are  being  erected.  The 
frame  of  the  dynamo  field  magnets,  which  compose  the  revolving 
part,  is  a  ring  of  forged  steel  made  by  the  Bethlehem  Steel  Company, 
by  the  same  process  which  is  used  by  them  in  making  armor  plate  for 
the  Government  men-of-war.  The  constructive  details  of  the  pole 
pieces  and  the  armature  have  not  yet  been  made  public  by  the  man- 
ufacturers and  therefore  cannot  be  described.  Fig.  321  shows  the 
way  in  which  the  dynamos  will  appear  when  entirely  completed. 

The  first  customer  to  which  electric  power  will  be  delivered 
when  the  great  dynamos  have  been  put  into  service  will  be  the 
Pittsburg  Reduction  Co.,  whose  works  for  the  production  of  aluminum 
by  electro-metallurgy  are  to  be  moved  from  Pittsburg,  Pa.  to  Niagara, 
in  order  to  take  advantage  of  cheap  electric  power.  It  is  expected 
that  other  manufactories  will  follow  and  that  quite  a  colony  of  large 
mills  and  factories  will  in  time  be  gathered  about  the  Niagara  electric 
power  house.  To  these  mills  it  is  proposed  to  distribute  the  current 
at  2,000  volts  pressure.  It  is  also  proposed  to  distribute  power  at  an 
early  date  to  factories  at  considerable  distances,  and  even  to  power 
users  in  the  city  of  Buffalo,  thirteen  miles  away.  After  a  time  it  is 
even  possible  that  power  will  be  furnished,  as  has  been  proposed, 
from  the  Niagara  plant  for  the  purpose  of  propelling  canal  boats  on 
the  Erie  canal,  and  for  manufacturing  purposes  in  cities  as  far  from 
Niagara  as  Rochester,  Syracuse  and  Albany.  For  the  transmission 
of  power  over  these  long  distances,  the  pressure  at  which  the  current 
is  supplied  to  the  lines  will  be  raised  by  means  of  transformers  from 
2,ooo  volts  to  10,000  or  20,000  volts  or  even  higher,  and  will  be 
reduced  to  a  safe  value  by  transformers  before  entering  the  consum- 
ers' premises.  Many  of  the  proposals  that  have  been  made  in  the 
newspapers  in  regard  to  the  transmission  of  power  from  Niagara  are 
manifestly  impractical,  but  many  of  its  possibilities  may  yet  be  unap- 
preciated, and  it  is  impossible  to  tell  what  developments  may  occur. 

Before  leaving  the  question  of  central  stations,  it  is  well  to 
examine  the  common  methods  of  handling  dynamos  in  a  plant 


290 


FIG.  317. 


291 


FIG.  319. 


FIG.   320. 

292 


FIG.   321. 


VJUL2JLJLJUL 
FIG.  322. 


293 


designed  to  furnish  electricity  for  lights  and  power.  As  has  already 
been  explained,  the  current  from  the  dynamos  is  led  to  the  switch- 
board by  conducting  cables  of  the  proper  size,  which  are  connected 
to  the  bus  bars  through  proper  indicating  instruments  and  switches. 
In  continuous  current  low  pressure  stations,  where  shunt-wound 
dynamos  are  used,  one  dynamo  terminal  is  usually  connected  directly 
to  the  proper  bus  bar  without  the  intervention  of  a  switch,  while  the 
other  dynamo  terminal  is  connected  to  its  bus  bar  through  a  single 
pole  switch.  In  alternating  current'  stations,  where  the  dynamos 
furnish  a  pressure  of  1,000  volts  or  more,  a  double  pole  switch  to 
which  both  cables  from  the  dynamo  are  connected  is  deemed  essen- 
tial. It  is  usual  to  operate  continuous  current  dynamos  in  parallel  on 
one  set  of  bus  bars,  but  alternators  are  almost  always  operated  on 
separate  circuits  in  this  country,  on  account  of  the  difficulty  of  keep- 
ing them  in  step  (L,esson  XXVII,  page  237).  This  makes  quite  a 
difference  in  the  arrangement  of  the  switchboards  in  the  two  kinds  of 
stations.  In  continuous  current  stations  all  feeders  are  connected 
directly  to  the  main  bus  bars,  but  in  alternating  current  stations  the 
feeder  switches  are  usually  arranged  so  that  any  feeder  may  be  indi- 
vidually connected  to  any  dynamo  as  desired. 

Fig.  315  shows  the  switch  board  of  a  3 -wire  Edison  incandescent 
lighting  station.  The  dynamo  regulators  are  shown  on  the  lower 
part  of  the  board  and  are  numbered  i,  2,  3  and  4  to  correspond  with 
the  numbers  of  the  four  dynamos  used.  Directly  above  these  are  the 
positive,  neutral  and  negative  bus .  bars.  Still  higher  on  the  board 
are  seen  four  ammeters  which  are  connected  to  the  four  dynamos  and 
which,  at  all  times,  tell  exactly  the  amount  of  current  being 
supplied  by  each  machine  to  the  bus  bars.  On  each  side  of  these 
ammeters  may  be  seen  two  sets  of  feeders,  each  set  having  three 
wires  which  are  connected  through  switches  and  cutouts  with  the 
three  bus  bars.  Between  the  feeder  switches  on  the  left  and  the  am- 
meters there  are  three  incandescent  lamps  arranged  in  the  form  of  a 
triangle.  These  constitute  the  ground  indicator  which  has  already 
been  described  in  Lesson  XXIV,  page  207. 

At  the  extreme  left  and  also  at  the  extreme  right  are  pressure 
indicators  and  multiple  arcing  galvanometers.  The  latter  are 
used  when  it  is  desired  to  cut  a  fresh  dynamo  into  the  circuit  and  by 
means  of  them  it  is  determined  when  the  pressure  of  the  dynamo  is 
the  same  as  the  pressure  of  the  bus  bars.  If  a  dynamo  were  cut  into 
the  circuit  at  a  time  when  its  pressure  was  not  equal  to  the  pressure 
of  the  circuit  there  would  be  a  flicker  or  jump  in  the  lamps  at  the 
moment  of  closing  the  switch  and  the  dynamo  would  take  either  a 
large  load  or  else  would  have  current  forced  into  it  so  that  it  would 
tend  to  run  as  a  motor,  depending  upon  whether  its  pressure  were 
higher  or  lower  than  that  of  the  bus  bars. 

One  of  the  pressure  indicators  is  connected  to  one  side  of  the 


three-wire  system  and  the  other  to  the  other  side  by  means  ot  a  series 
of  switches  located  beside  the  pressure  indicators.  The  switches  are 
so  arranged  that  the  indicators  may  be  connected  with  any  of  the 
centers  of  distribution  on  the  line  or  with  the  bus  bars. 

The  switches  for  cutting  the  dynamos  in  or  out  of  the  circuit 
are  not  shown  in  the  figure  but  are  located  upon  the  head  boards  of 
the  dynamos  as  shown  in  Fig.  304. 

We  will  suppose,  for  an  example,  a  large  continuous  current 
station  in  which  one  or  two  engines  with  their  dynamos  have  been 
running  all  day  to  supply  the  demand  for  current  in  the  daytime, 
and,  as  evening  approaches,  additional  engines  and  dynamos  must 
be  put  into  service  to  provide  for  the  greater  demand  for  current  dur- 
ing the  hours  of  dusk.  A  short  time  before  additional  machines 
are  likely  to  be  needed  one  or  more  engines  with  their  dynamos  are 
made  ready  for  running,  and  are  then  started  at  a  slow  speed  to  warm 
them  up.  After  a  time  one  of  the  sets  is  brought  to  full  speed  and 
the  dynamo  attendant  at  the  switchboard  changes  the  resistance  in 
the  field  circuit  by  means  of  the  dynamo  regulator,  which  is  placed 
on  the  board,  until  the  lamps  mounted  on  top  of  the  dynamo  burn 
with  approximately  normal  candle-power.  The  dynamo  is  then 
ready  to  be  put  into  circuit  whenever  it  is  needed.  When  this  time 
comes,  the  switchboard  attendant  connects  the  free  terminal  of  the 
dynamo  to  the  dynamo  galvanometer  (Fig.  322)  and  moves  the  dynamo 
regulator  until  the  galvanometer  needle  comes  to  zero.  The  pressure 
developed  by  the  fresh  dynamo  is  then  exactly  equal  to  the  bus  bar 
pressure.  The  attendant  now  closes  the  dynamo  switch,  thus  putting 
the  machine  into  circuit,  and  then  moves  the  regulator  until  the 
amperemeter  shows  that  the  dynamo  is  taking  its  proper  proportion 
of  the  load.  While  this  is  being  done,  another  generating  set  is 
brought  to  speed  and  made  ready  to  go  into  circuit  whenever  it  is 
required.  The  operation  is  repeated  until  all  the  dynamo  capacity 
that  is  required  during  the  period  of  heavy  load  is  in  service.  Some 
cities  are  subject  to  sudden  periods  of  darkness  caused  by  clouds 
or  smoke,  and  at  such  times  it  often  requires  very  prompt  action  on 
the  part  of  station  attendants  to  get  the  dynamos  into  circuit  as 
quickly  as  they  are  needed. 

After  a  period  of  heavy  load  is  over,  the  dynamos  are  withdrawn 
from  the  circuit  and  the  engines  shut  down.  When  a  dynamo  is  to 
be  withdrawn  from  the  circuit,  its  regulator  is  moved  until  the 
amperemeter  shows  that  it  carries  very  little  load  and  the  switch  is 
then  opened. 

The  process  of  getting  extra  dynamos  into  service  in  an  alter- 
nating current  station  is  quite  similar  to  the  preceding,  but  after  the 
dynamo  is  made  ready  to  receive  its  load,  it  is  not  put  in  parallel 
with  another  machine  but  one  or  more  feeders  are  transferred  to  it 
from  another  alternator  by  means  of  the  feeder  switches. 


29o 


The  arrangement  of  the  connections  in  an  electric  railway  power 
station,  where  compound  dynamos  are  used,  has  been  indicated  in 
the  preceding  lesson.  The  method  of  getting  compound  machines 
into  and  out  of  circuit  is  much  the  same  as  when  shunt-wound 
machines  are  used. 

Fig.  316  shows  the  electric  light  station  at  Tokio,  Japan.  We 
see  by  the  appearance  of  this  that  that  peculiar  nation,  the  Japanese, 
have  adopted  the  comforts  of  civilized  life  as  well  as  the  methods  of 
war  developed  by  civilized  nations. 

Copyrighted,  1895, 


The  National  School  of  Electricity. 

REVIEW   OF  LESSON  XXXI. 

Points  for  Review.     \.     How  long  has  it  been  since  electric  lighting  plants   became 
common  ? 

2.  When  was  the  first  central  station  put  in  operation,  and  where  ? 

3.  What   changes    have   been    made   in    the   general    mechanical   construction    of 
dynamos  in  the  past  ten  years  ? 

4.  Where  was  the  first  large  central  station  of  the  world  located  ?     How  long  ago 
was  it  started  ? 

5.  How  do  the  great  steam  dynamos  of  the  Pearl  street  station  compare  with  those 
built  now  ? 

6.  What  kind  of  electric  stations  were  built  after  the  ' '  Jumbo  "  type  of  dynamos 
were  abandoned  by  dynamo  manufacturers  ? 

7.  What  instruments  are  placed  on  a  station  switchboard  ? 

8.  What  is  the  object  of  the  third  bus  bar,  or  "equalizer,"  when  compound  dyna- 
mos are  connected  in  parallel  ? 

9.  What  is  the  object  of  "  multiple  arcing  galvanometers  "  or  "dynamo  galvano- 
meters," as  they  are  sometimes  called  ? 

10.  Why  are  double  pole  switches  always  used  on  the  switchboards  of  alternating 
current  power  stations  where  1,000  volts  are  used,  while  single  pole  switches  are  used  in 
stations  where  current  is  supplied  at  low  pressure? 

11.  Why  is  it  of  advantage  to  operate  dynamos  in  parallel? 

12.  Why  are  alternators  not  usually  operated  in  parallel? 

13.  How  are  the  feeders  usually  connected  in  a  continuous  current  station?     How 
in  an  alternating  station? 

14.  What  is  the  object  in  an  electric  station  of  starting  a  spare  engine  before  it  is 
actually  needed  for  service? 

15.  What  is  the  process  of  putting   a  dynamo  into  circuit  in  a  constant  pressure 
electric  power  station? 

16.  Why  is  it  necessary  to  have  the  pressure  of  the  incoming  dynamo  exactly  equaJ 
to  that  of  the  bus  bars  before  the  new  dynamo  is  connected  to  the  circuit? 

17.  What  is  the  process  of  cutting  a  dynamo  out  of  circuit? 


XXXII. 

UNDERWRITERS'  RULES,  ETC. 

The  importance  of  using  the  utmost  care  in  laying  out  and 
putting  in  place  the  electric  light  wires  which  go  into  houses  has 
already  been  explained  in  Lesson  XXIII.  It  now  comes  to  an 
explanation  of  the  more  important  rules  for  this  work  which  have 
been  issued  by  various  associations  of  underwriters  or  fire  insurance 
companies.  These  associations  issue  rules  for  carrying  on  electrical 
wiring,  and  in  the  large  cities  supervise  or  inspect  the  work  in  order 
that  danger  from  fire  may  not  be  introduced  into  buildings  insured 
by  them.  Many  of  the  chances  for  danger  which  exist  in  electric 
plants  are  caused  through  carelessness  or  lack  of  knowledge  and 

297 


experience  on  the  part  of  wiremen  who  may  be  employed  on  account 
of  the  false  economy  of  the  owner  of  the  plant.  In  electrical  work, 
as  in  much  else,  the  cheapest  is  by  no  means  always  the  be'st,  but  it 
is  often  difficult  to  make  this  fact  seen,  so  that  a  carefully  enforced 
set  of  rules  for  wiring  is  the  best  safeguard  which  the  owners  of 
buildings  and  the  underwriters  have  against  dangers  caused  by  care- 
less workmen  and  poor  workmanship. 

The  following  points  require  to  be  specially  looked  after: 

1.  That  the  general  workmanship  be  good,  and  especially  that 
joints  be  well  made  and  well  insulated. 

2.  That  the  conductors  have  ample  cross-section,  and  contain 
the  fewest  possible  joints. 

3.  That  the  insulating  material  on   the  conductors  be  of  the 
very  best,  and  that  the  insulation  resistance  of  the  completed  wiring 
be  sufficiently  high. 

4.  That  the  insulation  resistance  of  the  wiring  be  tested  from 
year  to  year  to  ascertain  whether  or  not  it  is  deteriorating. 

5.  That  all  constant  pressure  circuits  be  properly  protected  bv 
safety  fuses. 

By  insulation  resistance,  is  meant  the  resistance  as  measured 
from  either  conductor  of  the  plant  to  the  ground,  or  from  one  con- 
ductor to  the  other.  Practical  methods  for  making  insulation  tests 
have  already  been  explained  in  L,esson  XXIV.  The  actual  resist- 
ance of  the  insulation  of  the  wiring  in  any  particular  building,  will 
always  depend  upon  the  length  of  wire,  number  of  lamps,  and  char- 
acter of  the  fixtures  used  in  the  installation.  Thus,  for  instance, 
if  wire  is  used  which  has  an  insulation  resistance  of  1,500  megohms 
per  mile  and  ten  miles  are  used  the  total  insulation  of  the  wire  cannot 
be  expected  to  be  more  than  150  megohms,  while  if  only  two  or  three 
miles  of  wire  were  used,  the  total  insulation  resistance  might  be  ex- 
pected to  be  much  greater.  As  a  general  rule,  leakage  at  joints, 
lamp  sockets,  fuse  blocks,  and  fixtures  of  all  kinds,  has  a  much  more 
marked  effect  on  the  insulation  resistance  of  new  wiring  than  does 
the  leakage  through  the  covering  of  the  wire  itself,  so  that  the  under- 
writers require  these  points  to  be  specially  well  looked  after.  It  is 
usual  to  expect  a  much  higher  insulation  in  wiring,  before  the  sock- 
ets and  fixtures  are  connected  up  than  afterward,  and  in  some  places 
the  insulation  resistance  which  is  required  in  any  plant  is  allowed  to 
depend  upon  the  number  of  lamps  which  are  connected  to  the  wires. 

Unless  the  best  of  materials  and  workmanship  are  used  for  the 
wiring  put  in  a  building,  the  insulation  resistance  will  begin  to  fall 
within  a  few  months,  even  though  it  was  very  high  when  the  wiring 
was  first  put  it.  This  fall  in  the  quality  of  the  insulation  is  due  to 
several  causes,  chief  among  which  are  poorly  insulated  joints  and 
inferior  rubber  in  the  covering  of  the  wires.  Portions  of  wiring  which 
had  been  in  service  from  a  few  months  to  a  few  years  have  often 


298 


been  removed,  and  which  in  the  meantime  had  so  deteriorated, 
that  in  certain  spots  the  rubber  covering  on  the  wire  had  practically 
all  rotted  away.  It  is  sufficient  to  say  that  good  rubber-covered 
wire  does  not  act  in  this  way.  On  account  of  the  deterioration  of 
poor  material,  an  inspection  is  made  of  wiring  from  time  to  time  in 
some  cities,  and,  if  any  u  tap  "  falls  below  100,000  ohms  in  insula- 
tion measured  between  the  wires  and  the  ground,  or  between  the 
wires  themselves,  it  is  required  to  be  repaired. 

It  is  necessary  to  use  safety  fuses  on  all  constant  pressure  cir- 
cuits. Safety  fuses  must  be  of  such  a  capacity  that  they  will  blow 
or  melt  just  above  the  rated  carrying  capacity  of  the  smallest  wire 
which  they  protect.  It  is  customary  to  place  fuses  at  every  point 
where  a  change  is  made  in  the  size  of  wire,  excepting  where  small 
fixtures  or  drop  cords  are  attached  to  tap  lines.  L,esson  XXIII. 

The  rule  governing  the  minimum  number  of  lamps  ultimately 
dependent  upon  one  cut-out  varies  in  different  cities.  The  New 
York  rules  at  one  time  required  that  each  fixture,  even  if  it  were 
only  a  single  lamp  drop  cord,  must  be  connected  to  the  tap  line  by 
safety  fuses.  The  Chicago  rules  now  allow  groups  of  lamps  requir- 
ing five  amperes  to  be  operated  through  one  set  of  fuses.  All  motors 
must  be  protected  by  double-pole  cut-outs  and  controlled  by  double- 
pole  switches.  All  cut-outs  must  be  so  placed  that  they  can  be 
readily  seen  and  reached. 

In  general  the  size  of  a  fuse  depends  upon  the  size  of  the 
smallest  conductor  it  protects,  and  not  upon  the  amount  of  current 
to  be  used  in  the  circuit.  Below  is  a  table  showing  the  safe  carrying 
capacity  of  copper  conductors  of  different  sizes  in  Brown  &  Sharpe 
gauge  as  given  in  the  rules  of  the  Chicago  Fire  Department: 

TABLE  A.  TABLE  B. 

Concealed  Work.  Open  Work. 

B.  &  S.  G.  Amperes.  Amperes. 


0000  

...  218  

312 

000  

181  

262 

00  

150  

220 

0  

125  

185 

I  . 

105  

156 

2  

88  

I31 

3  

75  

no 

4  

63  

92 

5  

53  

.  ...  77 

6  

45  

65 

8  

33  - 

46 

10  

25  

32 

12  

17  

23 

14........ 

12  

16 

16  

6  

...  8 

18  

3  

.........  5 

299 


The  safe  capacities  given  here  are  greater  than  those  given  in 
some  rules,  but  experience  has  shown  that  they  are  amply  small  for 
real  safety,  provided  the  wiring  is  well  done  and  proper  sized  fuses 
are  used. 


Diagram  shotting  circuit  for  twio  Push  Buttons  for  &  single  Bell. 

X 


Diagram  anotuiog  circuit  tor  ringing  twuo  Bella  from  one  Push  Button, 
FIG.     334. 


FIG.  336. 


FIG.  335. 


300 


By  '  *  open  work ' '  is  meant  construction  which  admits  of  all 
parts  of  the  surface  of  the  insulating  covering  of  the  wire  being  sur- 
rounded by  free  air.  The  carrying  capacity  of  16  and  18  wire  is 
given,  but  no  wire  smaller  than  14  is  to  be  used,  except  for  fixture 
work. 

Until  comparatively  lately,  no  uniformity  existed  in  the  rules 
which  were  in  force  in  different  parts  of  the  country,  but  the  associa- 
tions of  the  underwriters  located  in  each  city  or  district  made  their 
own  rules.  This  resulted  in  much  annoyance,  and  did  not  tend  to 
produce  the  best  workmanship;  and  it  was  found  to  be  of  advantage 
to  formulate  a  satisfactory  set  of  rules  for  general  adoption,  which 
was  done.  These  rules  were  not  only  approved  and  adopted  by  the 
various  associations  of  underwriters,  but  were  also  approved  by  the 
two  large  associations  composed  of  the  officers  of  electric  light  com- 
panies— the  National  Electric  L,ight  Association  and  the  Association 
of  Edison  Illuminating  Companies. 

The  set  of  rules  thus  approved  are  now  generally  printed  by  the 
local  Boards  of  Underwriters  of  the  various  cities,  and  can  be  ob- 
tained from  their  inspectors.  In  some  cities,  the  city  authorities  con- 
trol this  matter  and  either  furnish  their  inspectors  with  the  generally 
adopted  rules  or  with  some  equivalent.  The  approved  rules  divide 
electric  light  and  power  circuits  into  six  classes,  i.  The  circuits 
inside  of  central  stations  and  the  dynamo  rooms  of  isolated  plants. 
2.  L,ines  constructed  for  the  purpose  of  operating  arc  lamps  in 
series.  3.  High  pressure  alternating  current  lines.  4.  L,ow 
pressure  continuous  current  lines,  and  low  pressure  inside  wiring 
(low  pressure  circuits  being  taken  to  include  all  circuits  on  which  the 
pressure  does  not  exceed  300  volts).  5.  Electric  railway  circuits. 
6.  Primary  and  storage  battery  circuits.  For  each  of  these  classes 
of  circuits,  special  rules  are  directed  towards  the  perfect  safety  of  the 
systems,  which  especially  emphasize  the  very  essential  points  ex- 
plained above.  Every  rule  has  a  good  reason  for  its  existence,  and 
experience  has  shown  its  propriety.  An  excellent  statement  of  the 
reasons  why  each  rule  should  be  carefully  carried  out  is  given  by 
Mr.  C.  C.  Haskins,  the  electrical  inspector  of  the  city  of  Chicago,  in 
a  series  of  articles  published  during  1895  ^n  tne  Electrical  Journal. 

The  only  classes  of  wiring  with  which  the  underwriter's  rules 
do  not  deal  directly  are  connected  with  telephone,  district  messenger 
call,  burglar  alarm,  electric  bell,  and  similar  systems  which  are  oper- 
ated by  electric  batteries.  Even  in  regard  to  these  wires  the  rules 
enjoin  proper  precautions  to  prevent  electric  light  and  power  wires 
from  becoming  crossed  with  the  poorly  insulated  battery  circuit 
wires. 

The  wires  used  for  these  battery  circuits  have  a  very  different 


301 


insulation  from  that  of  the  electric  light  wires.  The  wire  com- 
monly used  inside  of  buildings  for  such  circuits  is  called  "  annuncia- 
tor wire,"  which  is  a  copper  wire  with  an  insulation  consisting  of 
two  heavy  cotton  wrappings  wound  in  opposite  directions,  which  are 
thoroughly  waxed  and  paraffined.  These  wires  are  made  of  various 
colors  and  are  frequently  striped  in  different  colors.  Sometimes  what 
is  known  as  "office  wire"  is  used  for  telephone  and  messenger  call 
connections.  The  insulation  of  office  wire  ordinarily  consists  of  two 
braidings  of  cotton  well  soaked  in  paraffine. 

While  no  danger  can  arise  from  the  use  of  these  poorly  insulated 
wires  for  such  circuits,  provided  they  are  not  in  a  position  to  become 
crossed  with  electric  light  wires,  yet  a  great  deal  of  inconvenience 
may  be  caused  by  them.  For  instance,  in  Fig.  334  are  diagrams  which 
show  the  arrangement  of  electric  bell  circuits.  The  battery  consists  of 
one  or  two  open  circuit  cells,  which  are  connected  in  series  with  the 
bell  and  "push  button"  by  wires,  which  may  run  through  the  walls 
of  a  house.  When  the  button  is  pushed  it  closes  the  circuit  and  the 
bell  rings.  When  the  button  is  not  being  pushed  the  circuit  should 
be  open  and  the  battery  be  at  rest.  If  a  leak  occurs  from  wire  to 
wire  the  battery  remains  in  action  all  the  time,  and  the  depolarizer 
(if  the  battery  has  one)  soon  becomes  exhausted  and  the  battery 
becomes  polarized  or  "  run  down. "  The  bell  then  refuses  to  ring 
when  the  button  is  pushed.  If  the  battery  has  no  depolarizer  (L,esson 
IV,  page  24),  the  process  of  running  down  occurs  in  exactly  the  same 
way,  but  more  rapidly.  This  is  the  condition  of  numberless  elec- 
tric bell  circuits  in  houses  all  over  the  country  where  the  front  door 
bell  fails  to  ring  when  its  button  is  pushed.  The  trouble  is  caused 
by  the  current  leaking  from  poorly  insulated  wires  where  they  come 
in  contact  with  dampness  or  at  some  point  where  they  are  both 
placed  under  one  metal  staple,  and  the  difficulty  in  a  great  majority 
of  the  cases  would  never  have  appeared  had  wire  with  good  rubber 
insulation  been  used.  As  No.  18  B.  &  S.  wire  is  commonly  used  for 
bell  circuits,  the  extra  cost  caused  by  using  rubber  covered  or 
weather  proof  wire  is  not  very  great,  while  the  inconvenience 
avoided  by  its  use  may  be  considerable. 

It  must  not  be  assumed  that  all  the  trouble  to  which  bell  circuits, 
and  similar  circuits  are  heir,  comes  from  poor  insulation.  Battery 
zincs  become  used  up  or  the  water  evaporates  and  the  battery  may 
not  work  on  that  account.  The  mechanism  of  bells  and  push  buttons 
is  very  simple  and  not  likely  to  get  out  of  order,  but  trouble  may 
occur  even  in  them.  The  contacts  in  push  buttons  gradually  become 
corroded  and  when  the  button  is  pushed  it  does  not  complete  the 
circuit.  This  is  easily  remedied  by  taking  the  cover  off  the 
button  and  scraping  the  contact  points.  When  a  bell  gets  out  of 
order  a  little  testing  will  quickly  locate  the  trouble.  The  mechan- 
ism of  a  bell  consists  of  a  stationary  electromagnet  (Fig.  335)  with  a 


vibrating  armature  A  which  is  fastened  at  one  end  to  a  spring  hinge 
S  and  carries  at  the  other  end  the  bell  clapper  H.  When  an 
electric  current  is  passed  through  the  electromagnet  of  a  bell,  the 
armature  is  attracted  and  moves  forward  so  that  the  clapper  strikes 
the  gong.  At  the  same  time  the  electric  circuit  is  broken  by  a  spring 
contact  C  at  the  back  of  the  armature,  the  magnet  loses  its  mag- 
nesism,  and  the  armature  flies  back  to  its  original  position.  When 
the  armature  flies  back,  the  circuit  is  again  completed  at  the  spring 
contact,  the  armature  flies  forward,  the  clapper  strikes  the  gong,  and 
the  whole  process  is  rapidly  repeated  over  and  over  again  as  long  as 
the  electric  circuit  is  complete  at  the  push  button.  The  back  and 
forth  motion  of  the  armature  causes  the  clapper  to  strike  a  succes- 
sion of  blows  on  the  gong  and  thus  causes  the  ringing  of  the  bell. 
Wheti  a  bell  gets  out  of  order,  the  trouble  is  usually  to  be  found  in 
the  spring  contact,  which  may  be  dirty  or  out  of  adjustment,  or  the 
electromagnets  may  be  short  circuited.  Fig.  336  is  a  diagram  of  a 
push  button,  the  simplicity  of  which  may  be  seen  at  a  glance. 

It  is  an  interesting  fact  that  the  use  of  electric  bells  was  the 
first  application  of  electricity  to  household  purposes,  and  that  the 
principle  of  the  electric  bell  was  first  made  use  of  by  Prof.  Joseph 
Henry  about  1830. 

Copyrighted,  1895, 


303 


The  National  School  of  Electricity. 

REVIEW  OF   LESSON     XXXII. 

Points  for  Review:     1.     Why  are   "  Underwriters' Rules"  necessary? 

2.  What  points  must  be  specially  looked  after  in  electric  wiring? 

3.  Why  are  sockets,  fuses  and  fixtures  more  likely  to  allow  electricity  to  leak  than 
is  insulated  wire? 

4.  What  rules  are  now  generally  used  to  govern  electric  wiring? 

5.  Why  do  the  rules  separate  wiring  into  different  classes? 

6.  What  classes  of  wiring  do  hot  fall  directly  under  the  protection  of  the  Under- 
writers' Rules? 

7.  What  is  annunciator  wire?     What  is  office  wire?  • 

8.  Why  do  leaky  wires  cause  an  open  circuit  battery  to  run  down? 

9.  Where  can  copies  of  and  the  reasons  for   the  different   rules  adopted   by  the 
Underwriters  be  found? 


LESSON    XXXIII. 

ELECTRIC  WELDING,   FORGING,  ETC.    ELECTRICITY 
APPLIED  TO   THE  KITCHEN. 

The  use  of  the  electric  current  for  heating  and  working  metals 
is  not  new.  As  early  as  1865  patents  were  issued  relating  to  the 
subject,  but  on  account  of  the  great  expense  of  the  current  generated 
by  batteries,  these  early  endeavors  came  to  naught,  and  not  until 
within  a  very  few  years  has  electric  metal  working  been  made  an 
actual  success.  It  was  as  late  as  1888  before  electric  welding  was 
applied  to  commercial  uses,  but  immediately  upon  its  introduction  it 
came  rapidly  into  favor  and  even  created  much  excitement  among 
some  manufacturers. 

Electrical  methods  are  now  used  for  welding,  brazing,  heating, 
shaping  and  tempering  metals.  For  most  of  these  purposes  the 
method  in  common  use  is  to  pass  an  electrical  current  of  very  great 
volume  through  the  metal  to  be  worked.  This  great  current  gener- 
ates sufficient  heat  as  it  passes  through  the  resistance  of  the  metal  to 
quickly  raise  the  temperature  to  a  welding  or  bending  heat  or  even 
to  melt  the  metal.  This  method  of  heating  has  an  advantage  over 
the  ordinary  method  of  heating  in  the  forge  fire  which  heats  a  piece 
of  metal  from  the  outside,  while  the  electrical  method  heats  all  parts 
of  the  metal  equally  and  at  the  same  time  the  heated  metal  remains 
perfectly  clean.  The  apparatus  which  is  used  for  heating  usually 
consists  of  an  alternating  current  transformer  which  reduces  the 
pressure  of  an  alternating  current  from  300  volts  to  less  than  two 
volts,  and  increases  its  volume  proportionally  (Lesson  XXVII, 
page  235).  A  welder  transformer  is  shown  in  Fig.  337.  ^  The  grooved 
copper  casting  shown  in  the  figure  is  the  secondary  coil  of  the  trans- 


former  which  has  only  one  turn.  The  primary  winding  made  up  of 
numerous  turns  of  wire  is  intended  to  lie  in  the  groove  of  the  sec- 
ondary, while  the  core  which  is  seen  enclosing  one  side  of  the  second- 
ary casting  embraces  both  coils.  At  the  top  of  the  secondary  casting 
are  sliding  clamps  in  which  the  metal  to  be  heated  is  fastened. 

Electric  welding,  as  ordinarily  carried  on,  consists  of  heating, 
by  the  process  above  described,  nthe  pieces  of  metal  to  be  welded 
while  they  are  firmly  butted  against  each  other.  When  the  metals 
have  been  heated  till  they  are  soft  at  the  points  in  contact  they  are 
squeezed  together  a  certain  amount,  the  current  is  shut  off,  and  the 
weld  is  complete.  This  is  the  process  developed  so  usefully  by  Prof. 
Elihu  Thomson.  The  apparatus  which  is  generally  used  in  the 
Thomson  welding  process  is:  i.  An  alternator  usually  giving  a 
frequency  of  from  4x3  to  60;  2.  A  welding  transformer  with  clamps 
and  arrangements  for  automatically  making  the  welds;  3.  Apparatus 
for  controlling  the  amount  of  current  supplied  to  the  transformer. 
Fig.  338  shows  a  complete  Thomson  welder.  The  transformer  is 
seen  in  the  center  of  the  case  and  the  clamps  on  top.  The  weights 
at  the  left  are  for  squeezing  together  the  heated  rods  held  in  the 
clamps,  and  the  relay  shown  at  the  right  hand  side  is  for  cutting  off 
the  current  when  the  weld  is  completed.  In  welding  heavy  work, 
hydraulic  pressure  is  used  to  squeeze  the  weld,  as  shown  in  Fig.  339, 
which  is  a  welder  intended  for  electrically  welding  carriage  axles. 
In  Fig.  340  is  shown  an  arrangement  to  be  used  for  heating  pipe 
which  it  is  desired  to  bend.  The  transformer  is  seen  at  the  bottom 
of  the  figure,  and  the  clamps,  which  are  stationary  in  this  case,  are 
shown  holding  a  piece  of  pipe. 

Many  metals  may  be  welded  by  the  electrical  method  which 
cannot  be  coaxed  into  a  weld  by  the  ordinary  methods.  The  metals 
which  have  been  welded  by  the  Thomson  process  are  shown  in  the 
accompanying  table: 


Wrought  Iron 

Wrought  Copper 

Tin 

Cobalt 

Aluminum 

Gold  (Pure) 

Cast  Iron 

Cast  Copper 

Zinc 

Nickel 

Silver 

Manganese 

Malleable  Iron 

Lead 

Antimony 

Bismuth 

Platinum 

Magnesium 

Various  Grades 
of  Tool  Steel 

Musshet  Steel 

Wrought  Brass 

Fuse  Metal 

Aluminum  Al- 
loyed with  Iron 

Silicon  Bronze 

Various  Grade? 
of  Mild  Steel 

Stub  Steel 

Cast  Brass 

Type  Metal 
Solder  Metal 

Aluminum 
Brass 

Coin  Silver 

Steel  Castings. 

Crescent  Steel 

Gun  Metal 

Aluminum 
Bronze 

Various 
Grades  Gold 

Chrome  Steel 

Bessemer  Steel 

Brass 
Composition 

German  Silver 

Phos.  Bronze 

Again,  many  of  these  metals  may  be  welded  to  each  other  in 
combination.  The  combinations  which  have  been  made  are  shown 
in  the  table  below.  In  each  of  the  cases  where  a  weld  can  be  made 
at  all  it  becomes  practically  as  strong  as  the  metal  itself. 


COMBINATIONS. 


Copper  to 
Brass 

Brass  to 
Wrought  Iron 

Brass  to  Tin 

Wrought  Iron 
to  Tool  Steel 

Wrought  Iron  to 
Musshet  Steel 

Wrought  Iron 
to  Nickel 

Copper  to 
Wrought  Iron 

Brass  to  Cast 
Iron 

Brass  to 
Mild  Steel 

Gold  to  German 
Silver 

Wrought  Iron 
to  Stub  Steel 

Tin  to  Lead 

Copper  to 
German  Silver 

Tin  to  Zinc 

Wrought  Iron 
to  Cast  Iron 

Gold  to  Silver 

Wrought  Iron  to 
Crescent  Steel 

Copper  to 
Gold 

Tin  to  Brass 

Wrought  Iron 
to  Cast  Steel  x 

Gold  to  Platinum 

Wrought  Iron 
to  Cast  Brass 

Copper  to 
Silver 

Brass  to 
German  Silver 

Wrought  Iron 
to  Mild  Steel 

Silver  to 
Platinum 

Wrought  Iron  to 
German  Silver 

A  very  striking  application  of  electric  welding  has  been  adopted 
by  at  least  one  manufacturer  for  welding  together  the  parts  of  street 
railway  track  material,  such  as  switches,  frogs,  etc.,  which  are  ordin- 
arily made  up  by  bolting  together  pieces  of  rails  cut  to  proper  shape. 
By  the  welding  process  bolts  may  be  replaced,  and  the  work,  there- 
fore, is  made  much  more  substantial.  A  process  has  even  been 
developed  for  welding  the  rails  of  a  street  railway  track  together, 
thus  doing  away  with  the  usual  bolted  joints  which  cause  so  much 
roughness  in  the  track  and  require  such  a  large  expense  for  repairs. 
Fig.  341  shows  a  track-welding  outfit.  The  forward  car  is  equipped 
with  a  great  welder  shown  in  operation  in  Fig.  342.  This  welder  is 
arranged  to  work  on  track  which  is  in  place  in  the  street.  The  cur- 
rent is  supplied  to  it  by  a  rotary  transformer  which  transforms  the 
500-volt  continuous  current  taken  from  the  trolley  wire  into  an  alter- 
nating current  at  a  pressure  of  about  350  volts.  Fig.  343  shows  a 
complete  weld  at  a  rail  joint.  As  much  as  250  horse-power  is 
required  for  a  few  seconds  in  making  such  a  large  weld. 

One  of  the  striking  things  about  Thomson  electric  welders,  is 
their  ability  to  weld  up  rings,  so  that  they  may  be  used  in  welding 
wagon  tires,  chain  links,  etc.  In  this  case  the  question  occurs,  why 
does  the  current  not  flow  around  through  the  solid  metal  from  clamp 
to  clamp,  instead  of  through  the  path  where  the  ends  of  the  ring 
butt  against  each  other.  This  is  simply  a  question  of  electrical  re- 
sistance. In  the  case  of  a  wagon  tire,  the  electric  current  would 
have  to  flow  through  a  path  several  feet  long  in  going  from  clamp  to 
clamp  through  the  solid  metal,  while  the  path  through  the  point  to 
be  welded  is  only  a  few  inches  in  length,  so  that  the  latter  path 
is  of  much  the  least  resistance,  and  nearly  all  of  the  current  follows 
it.  In  very  small  rings,  enough  current  may  pass  between  the  clamps 


FIG.  337. 


FIG.  340. 


307 


OF  THE 

XTNIVERSIT 


FIG.  338. 


FIG.  339. 

3O8 


FIG.  341. 


FIG.  342. 

309 


FIG.  343. 


Sheet  iron  shield  to  prelect  workrnans  hands 


FIG.  344. 


310 


FIG.  345. 


FIG.  346  A. 


OF  THE 
XTNIVERSI 


311 


FIG.  347. 


312 


through  the  solid  part  of  the  ring  to  heat  it  red  hot,  but  that  does 
not:  interfere  with  the*  welding.  An  interesting  application  of  the 
Thomson  process  has  been  lately  made  to  softening,  at  points  where 
bolt  holes  must  be  drilled,  the  very  hard  nickel  steel  armor  plates 
which  are  made  for  United  States  men  of  war.  These  plates  are  so 
hard  that  it  is  almost  impossible  to  drill  them  as  they  come  from  the 
steel  works,  but  by  means  of  an  electric  heating  arrangement,  they 
are  softened  at  the  spots  where  the  bolt  holes  must  be  made  without 
injuring  the  temper  of  the  othea  parts  of  the  plates. 

Another  method  of  utilizing  the  heating  effect  of  electricity  ior 
the  purpose  of  welding  and  working  metals,  is  that  known  as  the 
arc  process.  This  was  first  used  by  De  Meritens,  a  Frenchman,  and 
was  later  more  fully  developed  by  a  Russian  named  Bernardos.  In 
this  process,  a  continuous  current  is  used  at  a  pressure  of  about  150 
volts,  one  terminal  of  the  electric  generator  being  connected  to  the 
metal  which  it  is  desired  to  heat,  and  the  other  terminal  being 
attached  by  a  flexible  conductor  to  a  portable  carbon  rod  (Fig.  344). 
When  the  carbon  rod  is  brought  against  the  work,  an  electric  arc  is 
set  up  and  the  metal  is  heated.  This  device  has  been  used  in  the 
process  of  filling  with  metal  blow  holes  which  occur  in  valuable 
castings.  It  has  also  been  used  for  welding  the  seams  in  small  iron 
boilers,  receivers  for  compressed  air,  etc.  It  is  of  special  advantage 
for  the  latter  work  since  the  arc  can  be  slowly  drawn  along  the  edges 
of  the  plate  to  be  welded,  thus  bringing  them  to  a  welding  heat,  and 
the  weld  is  then  completed  by  pressing  or  hammering  the  plates 
together. 

In  each  of  the  methods  of  electric  welding,  it  is  to  be  noticed 
that  the  electric  current  is  used  only  for  the  purpose  of  heating  the 
product  previous  to  welding,  and  that  the  pressure  required  to  com- 
plete the  weld  is  applied  by  mechanical  means  of  some  kind. 
There  is  another  striking  and  even  startling  method  of  electrically 
heating  metals  for  purposes  of  working  them.  If  a  pail  of  water  in 
which  is  dissolved  common  washing  sdda,  have  immersed  in  it  a  lead 
plate  which  is  connected  to  the  positive  terminal  of  a  150  or  200  volt 
electric  circuit,  it  gives  all  the  apparatus  necessary  for  quickly  heat- 
ing iron.  The  metal  to  be  heated  is  grasped  in  tongs  which  are 
electrically  connected  to  the  negative  terminal  of  the  electric  cir- 
cuit, the  handles  of  the  tongs  being  insulated.  When  the  metal  is 
plunged  into  the  pail  of  water,  it  is  quickly  brought  to  a  white  heat 
and  may  then  be  withdrawn  and  worked  on  the  anvil  or  welded  to 
another  piece  of  heated  iron^by  the  ordinary  blacksmith's  method. 
Any  metal  may  be  heated  by  this  process,  but  welding  can  be  per- 
formed only  on  those  metals  which,  like  iron,  may  be  welded  by  the 
blacksmith.  The  heating  of  the  metal  when  it  is  plunged  into  the 
water,  is  apparently  caused  by  an  electric  arc  which  is  set  up  around 


313 


die  submerged  metal  on  account  of  its  becoming  surrounded  by  a 
coating  of  hydrogen  gas.  The  amount  of  current  used  varies  from 
a  few  amperes  to  many  hundreds,  depending  upon  the  size  of  the 
metal  to  be  heated.  It  gives  one  a  remarkable  sensation  to  see  a 
piece  of  metal  which  is  dipped  into  a  pail  of  water  come  quickly  to 
a  blinding  white  heat,  and,  when  held  in  another  pair  of  tongs  (not 
connected  to  the  electric  circuit),  to  see  it  again  dipped  in  the  same 
water  for  the  purpose  of  cooling  it. 

The  direct  heating  effect  of  an  electric  current  as  it  passes 
through  resistance  coils,  is  now  applied  to  the  ordinary  purposes  of 
warming  and  also  to  cooking.  Fig.  345  shows  one  of  the  various 
forms  of  electric  heaters,  all  of  which  simply  consist  of  resistance 
wires  embedded  in  an  insulating  material.  These  heaters  have  been 
used  considerably  for  warming  electric  street  cars,  and  are  coming 
into  more  or  less  use  in  other  situations.  Electric  teakettles,  which 
are  kettles  with  an  electric  heater  in  their  base,  are  not  uncommon. 
Electric  flat-irons,  curling  irons,  soldering  irons  and  similar  devices, 
are  slowly  making  their  way  into  common  use  in  towns  where  incan- 
descent electric  light  circuits  are  at  hand  to  supply  the  necessary 
current.  In  Figs.  346,  A  and  B  are  shown  an  electric  saucepan  and 
an  electric  curling-iron. 

Whole  electric  kitchen  outfits  may  be  obtained,  including  an 
electric  range,  and  they  are  sure  to  come  into  quite  common  use 
when  their  cost  has  been  reduced  to  about  that  of  coal  ranges.  The 
arrangement  of  a  complete  electric  kitchen  outfit  is  shown  in 

Fig.   347- 

While  electric  cooking  can  be  said  to  be  commercially  satisfac- 
tory on  account  of  its  convenience,  cleanliness,  and  adaptability, 
electric  heating  for  general  purposes  can  never  replace  the  direct  use 
of  coal  or  the  use  of  steam  heating,  until  electricity  is  directly  gen- 
erated from  the  fuel  without  the  intervention  of  steam  engines  in 
which  enormous  losses  of  heat  are  absolutely  unpreventable.  The 
nature  of  steam  engines  makes  it  impossible,  even  with  the  best  of 
them,  to  convert  into  useful  power  more  than  ten  or  fifteen  per  cent 
of  the  heat  energy  contained  in  coal  which  is  shoveled  into  the 
boiler  furnace.  When  the  steam  generated  by  the  boiler  is  directly 
used  for  heating,  a  very  much  greater  proportion  of  the  heat  in  the 
coal  is  converted  to  a  useful  purpose.  The  electric  heater  can  never 
entirely  replace  direct  heating  by  stoves,  or  by  steam  pipes,  as  long 
as  the  generation  of  electric  currents  is  dependent  upon  steam 
engines. 

Another  electrical  device  jvhich  is  in  quite  common  use  is  the 
electrically  heated  flat-iron.  This  is  a  flat-iron  with  insulated  resist- 
ance wire  imbedded  in  its  interior,  very  much  in  the  same  manner 
as  the  resistance  coil  is  imbedded  in  electric  heaters.  These  irons 
are  much  used  in  laundries  because  they  can  be  kept  in  continuous 

214 


use  and  no  time  is  lost  by  the  ironers  in  changing  irons.  The  elec- 
trical irons  are  connected  to  the  electrical  circuit  by  means  of  a 
double  flexible  conductor,  so  that  the  current  can  reach  them  in 
whatever  position  they  stand.  Electrical  irons  are  also  used  a  great 
deal  in  private  houses,  because  they  can  be  so  conveniently  heated 
up,  and  they  can  be  kept  heated  without  requiring  the  presence  of  a 
hot  stove.  The  irons  must  be  carefully  used,  and  when  ironing  is 
not  being  done  with  them  the  current  must  be  turned  off  or  the 
resistance  wire  is  likely  to  be  burrled  out. 

Copyrighted.  1895, 


315 


The  National  School  of  Electricity. 

REVIEW  OF  LESSON  XXXIII. 

Points  for  Review.  1.  For  what  kinds  of  metal  working  are  electrical  methods 
nsed? 

2.     What  is  the  most  commonly  used  method  and  what  are  its  advantages? 

3^  Of  what  does  the  Thomson  apparatus  for  heating  metals  by  electricity  con- 
sist? 

4.  What  is  ordinarily  meant  by  electric  welding? 

5.  What  metals  may  be  welded  by  the  Thomson  method? 

6.  How  is  it  that  metal  rings  may  be  welded  on  a  Thomson  welder? 

7.  What  is  the  Bernardos  process  of  working  metals,  and  for  what  purposes  has  it 
been  used? 

8.  How  may  a  piece  of  metal  be  brought  to  a  high  temperature  by  dipping  it  into 
a  pail  of  water? 

9.  How  is  the  heating  effect  of  the  electric  current  used  for  warming  and  for  cook- 
ing? 

10.  Why  is  it  not  possible  at  present  for  electric  heating  to  replace  the  use  of 
stoves  for  heating? 


LESSON   XXXIV. 

ELECTRO  THERAPEUTICS. 

One  of  the  first  practical  uses  ever  made  of  electricity  was  in  the 
treatment  of  disease.  Its  application  at  the  beginning,  however, 
was  entirely  experimental  and  the  cures  achieved  through  its  agency 
were  largely  matters  of  accident,  as  it  was  used  indiscriminately  and 
for  all  pathological  conditions.  Static  electricity  or  Franklinization 
was  almost  exclusively  employed,  not  especially  because  it  was  be- 
lieved to  be  better  than  any  other  variety  of  the  current  but  for  the 
reason  that  there  was  no  other  form  of  controllable  electricity  in 
existence  at  that  time.  Electricity  in  this  field  followed  the  course 
pursued  by  nearly  all  other  therapeutic  agents  upon  their  introduc- 
tion. It  was  used  for  every  disease  under  any  circumstance  by  the 
most  ignorant  people;  indeed  nothing  was  known  of  its  physiological 
action  and  long  years  ago  its  use  would  have  been  prohibited  by 
popular  opinion  but  for  the  fact  that  since  the  ancient  Phoenicians 
generated  electricity  by  rubbing  a  piece  of  amber,  calling  the  pro- 
duct a  reanimated  soul,  the  mysteries  of  electricity  have  been  some- 
thing to  conjure  with.  The  result  of  the  first  few  years'  use  of  elec- 
tricity in  medicine  demonstrated  its  popular  approval  and  the  result 


was  that  large  numbers  of  ignorant  and  incompetent  people  set 
themselves  up  as  electrical  doctors  and  a  most  natural  consequence  was 
that  in  the  legitimate  profession  of  medicine  electricity  came  into 
disrepute.  A  further  sequence  was  that  these  medical  quacks  were 
given  free  and  undisturbed  occupation  of  the  field  to  the  exclusion 
of  scientific  men  who  would  have  got  out  of  electricity  all  that  there 
was  good  in  it,  and  this  unhappy  condition  obtained  until  a  very  few 
years  ago  when  a  few  members  of  the  profession  became  brave  enough 
to  risk  the  condemnation  of  their  fellows  by  making  exploratory 
incursions  into  the  so-called  mysteries  of  electricity.  Such  investi- 
gators are  becoming  more  numerous  day  by  day  and  it  is  safe  to  say 
definitely  that  electricity  is  rehabiliated  in  the  profession  of  medicine 
and  that  it  has  taken  its  place  with  other  powerful  agents  when 
applied  in  the  hands  of  scientific  men.  One  of  the  principal  reasons 
why  electricity  has  not  been  better  understood  in  the  profession  of 
medicine,  is  that  medical  men  as  a  rule  are  not  physicists  and  they 
have  not  mastered  the  physical  laws  of  electricity  and  hence  are 
not  able  to  investigate  as  to  its  physiological  action  under  circum- 
stances likely  to  be  met  with  in  medical  and  surgical  practice.  The 
few  brave  men  mentioned  above  have  recognized  this  weak  position 
and  they  have  gone  to  work  earnestly  to  master  electro-physics  and 
their  work  brings  us  down  to  the  present  day  and  to  a  clear  judg- 
ment as  to  what  electricity  will  do  in  medicine  and  surgery. 

According  to  the  circumstances  of  its  application,  its  physiolog- 
ical action  or  its  purpose  in  the  treatment  of  disease,  electricity  is 
divided  into  the  direct  or  continuous  or  galvanic  current,  the  alter- 
nating, or  to  and  fro,  orfaradic  current,  and  static  electricity  or  elec- 
tricity at  rest. 

The  Direct  Current  in  Therapeutics. — The  action  of  the  direct 
current  in  its  application  in  therapeutics  may  be  arranged  in  four 
principal  divisions  according  to  the  use  to  which  it  is  put;  first,  elec- 
trolysis; second,  cataphoresis ;  third,  the  electro-cautery,  and  fourth, 
the  incandescent  lamp  for  purposes  of  diagnosis  in  the  exploration  of 
cavities. 

Electrolysis,  as  applied  in  therapeutics,  does  not  differ  from  the 
ordinary  chemical  electrolysis  of  which  we  have  learned  far  back  in 
the  course,  but  there  are  physiological  changes  that  must  be  under- 
stood in  order  to  appreciate  the  conditions  under  which  electrolysis 
can  be  applied  in  the  healing  of  disease.  We  have  learned  that  in 
the  electrolytic  decomposition  of  water,  hydrogen  collects  at  the 
negative  or  cathodal  electrode,  and  oxygen  at  the  positive  or  anodal 
electrode.  This  is  also  true  when  electrolysis  is  applied  upon  the 
human  tissues,  but  further  than  this,  the  acids  of  the  tissues  are 
attracted  to  the  anode  or  positive  electrode,  and  alkalies  to  the  oppo- 
site pole.  The  simple  statement  of  these  facts  carries  with  it  appre- 


ciation  of  the  conditions  calling  for  the  use  of  these  two  forms  of 
electrolysis.  In  anodal  electrolysis  the  first  effect  is  exactly  the 
effect  of  acid  applications,  that  is,  first  astringent,  second  styptic, 
third  caustic.  The  cathodal  or  negative  effects,  on  the  contrary  are 
similar  to  those  occurring  upon  the  presence  of  alkalies  in  the 
tissues,  that  is,  there  is  a  softening  of  all  the  tissues,  or  a  disposition 
on  the  part  of  the  tissues  affected  to  become  pulpy  and  consequently 
yielding.  Tf  we  take  a  piece  of  raw  meat,  for  instance,  large  enough 
for  clear  demonstration,  and  insert  a  positive  needle  electrode  at  one 
point  and  the  negative  at  an  opposite  point,  and  apply  a  few  milliam- 
peres  of  current,  and  if  this  action  be  allowed  to  operate  for  a  few 
minutes,  we  will  find  upon  investigation  that  the  meat  at  and  about 
the  anode  has  become  hardened  and  dry  and  contracted,  due  to  acid 
influence,  while  the  tissues  at  and  about  the  cathode  have  become 
soft  and  more  pulpy  than  they  were  before.  The  anodal  effect  is 
produced  by  a  coagulation  of  the  albuminoids  in  the  tissues,  due  to 
acid  reaction.  In  therapeutics  these  two  forms  of  electrolysis  are 
clearly  defined  and  will  be  applied  under  opposite  conditions;  cathodal 
or  negative  electrolysis  will  be  indicated  for  the  softening  of  connec- 
tive tissue  or  scars,  for  instance.  More  specifically,  it  may  be  used 
for  example,  to  soften  scar  tissue  resulting  from  an  attempt  on  the 
part  of  a  person  to  swallow  some  active  caustic  such  as  lye,  where 
the  throat  and  oesophagus  have  been  badly  burned  and  where  scar 
tissue  has  subsequently  formed.  During  the  application  of  the  cur- 
rent, in  which  the  cathode  will  be  applied  as  nearly  as  possible  to  the 
part  affected  and  the  anode  remotely,  the  scar  tissue  will  become 
softened  in  a  marked  degree,  and  at  the  end  of  a  few  minutes'  appli- 
cation, a  bougie  or  dilator  may  be  pasred  that  it  was  impossible  to 
pass  before  the  application,  and  if  this  course  of  treatment  is  con- 
tinued at  intervals  of  once  a  week  it  will  be  found  that  on  each 
application  the  calibre  of  the  canal  will  be  materially  increased  in 
size,  and  a  larger  sound  can  be  passed  at  the  end  of  each  electrical 
application ;  so  that  a  course  of  treatment,  extending  over  some  weeks, 
will  result  in  the  restoration  of  the  oesophagus  to  nearly  its  normal 
size.  The'  principle  involved  in  this  treatment  can  be  applied  indefi- 
nitely under  a  variety  of  circumstances  where  the  pathological  condi- 
tions are  like  those  in  the  case  cited. 

Naturally  anodal  or  positive  electrolysis  would  be  applicable  in 
the  contrary  conditions  such  as  the  naevi  or  birth  marks  that  so  dis- 
figure the  face  and  that  occur  frequently  in  other  parts  of  the  body, 
or  other  forms  of  abnormal  growth  possessing  much  vascularity. 
In  the  destruction  of  these  growths  the  thermo-cautery  used  to  be 
applied  and  invariably  left  scars  sometimes  more  unsightly  than  the 
original  growths.  The  knife  has  also  been  employed  with  equally 
disfiguring  effect.  In  employing  anodal  electrolysis  the  desire  is,  of 


313 


course,  to  cut  off  the  circulation  that  supplies  the  growth  or  that 
reddens  the  naevus  mark,  and  by  using  fine  needle  electrodes  either 
one  at  a  time  or  a  number  together,  with  the  anode  or  positive  elec- 
trode at  the  point  of  attack,  and  the  negative  remotely,  the  albumin- 
oids in  the  blood  are  coagulated  and  the  blood  supply  practically  cut 
off,  but  it  is  done  so  gracefully  and  with  so  little  destruction  of  tissue 
that  no  scars  result  and  the  naevus  part  becomes  the  color  of  the 
surrounding  skin,  and  in  the  case  pf  the  polypi  and  fungous  growths 
the  circulation  is  cut  off  and  they  disappear,  leaving  no  scar  or  at 
most  a  small  point. 

Cataphoresis  or  Transfusion  or  Electric  Osmosis. — If  two  fluids 
are  separated  by  a  membrane  or  consistent  partition, 'and  the  direct 
current  of  a  few  milliamperes  is  passed  from  one  fluid  to  the  other, 
the  fluid  in  that  compartment  connected  with  the  positive  electrode 
or  anode,  will  be  found  to  decrease  in  quantity,  and  that  connected 
with  the  cathode  or  negative  electrode,  to  increase.  In  other  words, 
the  current  has  the  effect  of  carrying  one  fluid  through  the  partition 
to  the  other,  the  direction  of  conveyance  being  from  positive  to 
negative.  Reverse  the  current  and  the  bulk  of  the  fluids  is  changed 
again.  This  is  simply  the  principle  of  osmosis  as  applied  in  physics. 
We  have  a  collection  of  fluid  in  the  synovial  sac  of  the  knee  joint 
and- we  desire  to  be  rid  of  it.  Pressure  and  other  means  have  failed 
but  we  will  find  after  experiment  that  the  dissipation  of  the  fluid  will 
be  materially  hastened  if  not  actually  brought  about  by  this  form  of 
current  intelligently  applied;  or  we  have  an  accumulation  in  the 
pleural  sac,  or  a  pericardial  effusion,  that  may  be  dissipated  in  the  same 
way.  It  may  not  be  said  positively  that  the  application  of  this  princi- 
ple will  invariably  remove  these  fluids,  but  conditions  will  be  very 
favorable  to  their  removal,  and  nature  and  other  agents  may  be  greatly 
aided.  This  principle  is  reversible  and  may  be  applied  from  without 
internally.  We  wish  to  administer  medicines  that  it  is  not  expedient 
to  give  by  the  mouth  or  by  means  of  the  hypodermic  needle.  Solu- 
tions of  the  proper  agent  on  a  sponge  electrode,  the  particular  elec- 
trode being  intelligently  chosen  according  to  the  reaction  of  the 
agent,  and  applied  at  a  point  on  the  skin  nearest  that  desired  to  be 
reached,  with  the  opposite  electrode  remotely  attached,  is  a  thoroughly 
practicable  and  serviceable  method  of  administration.  L,ocal  anaes- 
thesia may  be  brought  about  in  the  same  way  and  this  method  of 
administration  will  be  found  an  extremely  desirable  one  where  it  is 
required  to  operate  within  confined  localities  or  in  very  small  com- 
pass, such  as  medicinal  applications  for  intra-uterine  inflammations 
or  inflammatory  conditions  of  the  nasal  fossae,  or  the  throat. 

In  electrolysis  and  cataphoresis  we  have  been  considering  the 
applications  of  electricity  in  the  treatment  of  disease  where  the  cir- 
cuit; was  partly  made  up  of  human  tissue,  and  where  the  effect  de- 


319 


sired  to  be  obtained  was  brought  about  by  changes  effected  in  the 
tissues.  We  come  now  to  the  application  of  electricity  where  the 
tissues  of  the  body  are  not  involved  in  so  far  as  the  circuit  is  con- 
cerned. These  applications  are  the  electro-cautery  and  the  electric 
light,  the  latter  used  mostly  for  the  illumination  of  cavities  desired 
to  be  explored  for  purposes  of  diagnosis.  In  neither  of  these  forms 
of  the  application  of  the  electrical  current  are  we  confined  to  the 
use  of  the  direct  or  continuous  current,  for  the  alternating  current 
may  be  used  instead  wherever  it  is  convenient  or  obtainable,  but  as 
the  alternating  current  has  so  recently  come  into  use  for  these  pur- 
poses, and  as  physicists  and  manufacturers  have  hardly  perfected  the 
necessary  apparatus  for  its  use  in  these  connections,  it  may  be  well 
to  consider  this  work  only  in  relation  to  the  direct  current. 

Electro  Cautery. — We  have  seen  that  when  a  direct  current  is 
flowing  through  a  circuit,  the  amount  of  current  at  every  point  in 
the  circuit  is  the  same,  and  the  heating  effect  of  a  current  at  any 
point  in  its  circuit  is  dependent  upon  the  resistance  at  that  point. 
If  the  circuit,  the  main  part  of  which  offers  but  little  or  no  resistance, 
is  partly  composed  of  a  substance  having  a  comparatively  high  resis- 
tance, the  amount  of  heat  that  will  be  generated  in  this  part,  will  be 
in  direct  proportion  to  the  square  of  the  quantity  of  current  that  passes 
through  it  in  a  given  unit  of  time.  This  quantity  may  be  so  great  as  to 
generate  sufficient  heat  to  render  the  resistant  part  of  the  circuit  incan- 
descent, and  when  this  is  the  case  we  have  the  essential  necessary  to 
the  heating  of  a  knife  or  burner  for  cautery  work.  The  sources  from 
which  current  may  be  derived  for  cautery  work  are  the  primary  bat- 
tery, the  secondary  battery  and  the  dynamo;  but  it  will  hardly  be 
profitable  for  us  to  recapitulate  the  details  incident  to  the  use  of  these 
forms  of  current  as  we  have  already  been  over  that  ground  suf- 
ficiently to  be  familiar  with  it.  It  may  be  said  that  the  electric  cautery 
has  marked  an  evolution  'or  distinct  epoch  in  surgical  procedure. 
The  thermo-cautery  formerly  in  use,  had  the  disadvantage  of  ex- 
treme irregularity  of  temperature,  and  it  was  almost  impossibe  to 
regulate  the  temperature  of  a  thermo-cautery  knife,  so  that  one  could 
always  be  sure  of  obtaining  the  grade  of  cautery  action  desired.  If 
we  require  a  cutting  effect,  and  do  not  fear  bleeding,  we  may,  with 
the  electro-cautery,  have  the  knife  at  a  white  heat.  If  we  prefer 
to  create  an  eschar  as  we  progress,  so  that  bleeding  will  not  take 
place,  we  may  have  the  cautery  at  a  dull  red  heat,  and  if  we  do  not 
require  a  cutting  action  at  all,  but  simply  a  styptic  action,  the  heat 
may  be  still  more  moderate;  moreover  the  electro-cautery  has  the 
added  advantage  over  the  ordinary  thermo-cautery,  in  that  the  wire 
electrode  or  knife  or  burner  may  be  introduced  while  cold  into  a 
cavity  arid  directly  down  upon  the  point  of  attack,  and  be  heated  to 
the  required  intensity  while  in  position.  It  will  not  be  necessary  to 


320 


detail  cases  where  the  electro-cautery  is  indicated.     These  will  be 
apparent  to  the  medical  man  without  discussion. 

The  Electric  Light. — The  extreme  daintiness  of  manufacture 
and  gracefulness  of  the  mechanism  now  turned  out  by  manufacturers 
makes  it  possible  to  achieve  some  beautiful  results  with  sounds  in 
combination  with  mirrors  placed  at  proper  angles,  and  little  lamps 
to  light  the  cavity  at  the  end  of  the  sound.  These  instruments  are 
made  for  optical  inspection  of  the  cavities  of  the  body  such  as  the 
stomach,  bladder,  throat,  rectum.  As  a  rule  the  cavity  intended  to 
be  explored  is  partially  or  wholly  filled  with  distilled  water  in  order 
to  protect  the  membranes  from  the  heat  of  the  lamp.  By  the  careful 
and  scientific  use  of  such  mechanism  as  this  we  are  able  to  discover 
and  locate  stone  in  the  bladder  or  ulcerations  or  disease  of  any 
character  upon  the  internal  mucous  membrane  of  the  cavities  or 
canals. 

Indiiced  Currents  and  Static  Electricity . — These  forms  of  electric 
action  are  now  also  extensively  used  in  therapeutic  applications  and 
produce  effects  differing  according  to  the  nature  of  the  mechanism 
used  to  produce  them.  Induction  coils  can  be  so  wound  as  to  vary 
the  E.M.F.  over  very  wide  ranges,  and  the  interruptions  of  the  pri- 
mary current  may  likewise  be  varied,  and,  according  as  these 
are  fast  or  slow,  the  physiological  and  therapeutic  effects  differ.  The 
medical  induction  coil  is  a  very  convenient  and  serviceable  instrument 
for  arousing  torpid  physiological  action  and  bringing  into  play  latent 
energy.  But,  as  its  currents  are  small  in  quantity,  as  compared 
with  direct  currents,  and  are  either  intermittent  or  alternating,  the  elec- 
trolytic and  cataphoric  effects  are  very  inconsiderable  in  comparison. 
The  high  E.M.F.  of  these  currents  render  them  very  stimulating,  but 
the  secondary  current  may,  by  great  frequency  6f  interruptions,  be 
made  to  act  as  a  sedative  to  irritated  sensory  nerves. 

Static  electricity  possesses  still  higher  E.M.F.  and  much  less 
current  than  that  derived  from  induction  coils,  but  it  has  been  found 
to  exercise  a  marked  influence  upon  molecular  action  in  the  tissues 
of  the  body  and  is  a  powerful  modifier  of  nutrition.  It  is  used  to 
the  best  advantage  in  cases  of  malnutrition  and  functional  disorders, 
especially  those  affecting  the  nervous  system. 

The  instruments  and  apparatus  for  the  application  of  electricity 
in  its  varieties  of  form,  and  for  the  various  therapeutic  uses,  are  so 
great  in  number  and  of  such  variety  that  it  would  not  be  expedient 
to  enter  upon  their  discussion,  and  it  need  be  said  only  that  the 
mechanism  involved  in  this  sort  of  apparatus  does  not  differ  in  any 
degree  in  principle  from  the  instruments  and  apparatus  used  in  the 
ordinary  commercial  applications  of  electricity,  and  the  only  differ- 
ence will  be  found  in  the  direction  of  mechanical  construction. 

Copyrighted,  1895, 


321 


INDEX. 


PAGE. 
Acid  radical 92 

Air  gap 158 

Alternating  currents 151,  -220 

Ammeters 75 

Ampere 12,  44 

Ampere  hour 86 

Ampere's  rule 37 

Ampere  turn 39 

Anode 63 

Apparent  resistance 227 

Arc  lights 167 

Arc  switchboards 176 

Armatures 152 

B 

Batteries 15 

Primary   15 

Storage 25 

Bipolar  machines 165 

Brushes 152 

Brush  holders 166 

Built-in  system  of  wiring 193 

Bunsen's  photometer 210 


Candle  power 210 

Candle  foot 211 

Carrying  capacity  of  wires 299 

Cathode 63 

Calorimeter 53 

Circular  mils 184 

Coercive  force 31 

Collecting  rings 152 

Commutators 152 

Compound  winding 160 

Condensers 87 

Conduits...  ,.  132 


PAGE. 
Conductivity 4 

Conductivity  tests 138 

Conductors 4 

Counter  electromotive  force 160 

Controllers 276 

Coulomb 7 

Coulombmeters 86 

Coupling  box 194 

Crosstalk 140 

D 

Dieletric 87 

Differential  magnets 172 

Divided  circuits 46 

Divided  wire  bridge 71 

Drop,  method  of  finding 185 


Earth  currents 142 

Edison  tubing., 194 

Eddy  currents 157 

Effective  current 223 

Effective  pressure 223 

Electricity 1 

Statical 2 

Current 2 

Electric  arc 167 

Cooking 314 

Launches 292 

Locomotives 265 

Plants 279 

Railways , 256 

Smelting 101 

Welding 305 

Electrical  capacity 86 

Potential 13 

Electro-chemical  equivalent 24,  65 

Electro-dynamometers 78 


PAGE. 

Electrolytic  copper 99 

Electrolysis 63,  100 

Electrolytes 63 

Electromagnet 40 

Electrometers 8 

Electromagnetic  induction 122,  213 

Electromagnetism 36 

Electrophorus 10 

Electroplating 91 

Electroscope 5 

Electro  therapeutics 316 

Electrotyping 98 


Fan  motors,  etc 242 

Farad 86 

Feeders 196 

Fish  plates 264 

Five-wire  system „ 189 

Frequency 224 

G 

Galvanometers 58 

Ballistic 88 

Tangent 59 

Reflecting 59 

Sine 59 

Astatic 61 

D'Arsonval 62 

Constant  of 62 

Geissler  pump 178 

Ground  detector 208 

H 

Heat  loss 53 

Holtz  machine 10 

Hot  wire  instruments 78 

Hysteresis 157 


PAGE. 

Joule 51 

Junction  boxes 196 

K 

Kelvin  balance 219 

sockets 181 


Leaks  and  Breaks 138 

Lenz's  law , 217 

Leyden jar 90 

Lines  of  force 34 

M 

Magnetic  field 32 

Magnetic  pole 32 

Magnetic  conductivity 41 

Magnetic  induction 29  147 

Magnetic  storms 142 

Magnetism 27 

Magnetos 147 

Magneto  motive  force 41 

Magnets 28 

Mclntyre  joint 128 

Measurement  of  light 205 

Megohm 72 

Microamperes. 75 

Microphone 121 

Milliamperes 75 

Morse  alphabet , 106 

Multiple  arcing  galvanometers 294 

Multipolar  machines 165 

N 

Neutral  wire 187 

Nickel  solutions 97 


Incandescent  lamps 178 

Induced  currents 214 

Induction 4 

Induction  coil 214 

Induction  motors 238 

Inductive  capacity 88 

Inside  wiring 198 

Insulators 4 

Insulation  tests 138 

Ions 63 

Ironclad  motors...,                             ....  165 


Oersted's  experiment 35 

Ohm 44 

Ohm's  Law...  ,     42 


Paramagnetic 30 

Permeability 40 

Polarization 19 

Polyphase  machines 238 

Poor  connections. ...  ,.  138 


PAGE. 

Potentiometer 83 

Power  loss  in  transmission 253 

Pressure  indicators 80 

Pressure  wires 196 

Primary  coil 214 


Rail  bonds 264 

Recording  voltmeter 266 

Regulators 161 

Rheostats 66 

Reluctance 41 

Residual  Magnetism "...     40 


Secondary  coil 214 

Self  induction 218-226 

Series  winding 160 

Shunt  winding 160 

Silver  solutions 93 

Sludge 100 

Solenoids 39 

Span  wires 264 

Sparking 166 

Spark  coils 218 

Specific  heat 55 

Sprengle  pump 180 

Spring  jack 134 

Squirrel  cage  armature 239 

vStarting  boxes 190 

Statical  electrictity 2 

Law  of  attraction 3 

Synchronism 236 


Telegraph 

Relays 108 

Duplex 108 


PAGE. 

Diplex 108 

Multiple HO 

Automatic 117 

Autographic 117 

Sounder 103 

Line 103 

Telephone 118 

Receiver 118 

Transmitter 118 

Line 125 

Switchboard 134 

Temperature  coefficient 49 

Testing  power  circuits 191 

Testings  sets 70 

Testing  submarine  cables :..  145 

Three-phase  machines.... 237 

Three- wire  system. 187 

Trailers 258 

Transformers 216-231 

Capacity  of 235 

Trolley  wire 257 

Two-phase  machines 237 


U 


Underwriters'  rules. 


297 


V 


Volt 13 

Voltameters 62 

Voltmeters 80 

Electrostatic 82 

W 

Wattmeters .  84 

Watt  hour 85 

Weather-proof  wire 192 

Wheatstone  bridge 68 


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